Have you been down this road? Any advice? Am I horrible?

Okay, here's my take with the additional information you gave us.  There was something about your wife that made you fall in love with her...maybe that she's a nice person, I don't know what it was.  You did make a mistake in thinking that she would change into a different person as time went on.  May be a little late on the advice here, but never, ever expect to marry someone in the hopes that they'll change over time to fit your expectations or desires.  They won't.  If they change, it's likely to be because of something other than your desires and it likely won't be a change that you'll like either.  If you love someone, you have to love them for who they are, not who you would wish them to be...you have to accept their flaws and imperfections along with the reasons you fell in love with them, or you need to say goodbye if you can't handle their flaws.

Now, it could be that this person is different, new, exciting, and may seem like a match for what you feel is missing in your life.  But what about her flaws and imperfections?  Do you know what they are yet?  Is the grass really greener on the other side of the fence?  Something to think about...it could very well be that the more you get to know this person, you could find that she isn't what you want after the "novelty" of the newness of a relationship wears off.  Then again, she could be exactly what you're looking for, too.  Is it a gamble that you're willing to take?

Only you can decide that.  It's not my place to tell you your feelings are right or wrong...only you can do that.  What I can tell you is that you need to figure out what it is that you want in life and then you need to tell your wife, first.  If she is willing to go with you on your journey, then stay married.  If your goals are no longer compatible, then you need to decide, together, how best to resolve that...divorce, if needs be.

Good luck to you, in whatever you decide.

You are not a horrible person.  Sorry if this is a bit long-winded, but my situation is very similar to yours.  I have to agree with everyone who told you to do all that you can to make things work with your wife.  Only you will know when it is time to go, if you should decide that.  Putting things on hold with the other lady is also a good idea.  I met a wonderful guy where I used to work and I have not been happy in my marriage for quite some time.  I struggle with how much this will hurt my husband, but we have lived liked roommates for so long.  I can easily picture a future with the other person, but any of those possibilities are on hold now.  I got transferred to work somewhere else, so I won't have to see him all the time.  It was just too distracting and confusing to see him for 8 hours and then go home and try to work things out with D.  sort of like trying to lose weight and spending hours in a donut shop.  Hanging out with or talking to the other lady and trying to save your marriage will not go well together.  I already made my decision and now it's just a matter of saving up money for the lawyer.  I am telling you all of this so you know that I am sympathetic to your dilemma and do not judge you.  I asked myself if I could take another 30 or 40 years of life with my husband and I just knew I couldn't do it.  As a friend of mine pointed out, guilt is a lousy reason to remain in a relationship.
      If you do decide to end your marriage and things do not work out with the other lady, you just might be better off on your own.  I had to picture that scenario for myself.  What if he won't wait for me?  Turns out that he did not and has been dating a woman for 3 months now.  It hurts but it helped me realize that I still want out of this marriage.  I would rather be alone then feel trapped in an unhappy situation. 

Thank you for your responses. There was a very common theme, and honestly I suppose I could have predicted that theme before I read. We have had conversations about my needs vs. hers. The problem is that she is usually very happy with the way things are. She is perfectly content to work all day and read a book all evening before going to bed before 10. She is a good mom and a nice person. When we discuss how I want to experience more of life than just work and sleep, she simply doesn't understand my perspective. It is really my fault no matter how you slice it. The problems lie within me...and this "expectation" gap has always been there, but I always thought she would change. She will say that she is willing to go out and do things, but when we do, she typically sits aside from others and watches the clock as if she's putting in her hours at a factory. Then I feel like a jerk for putting her in uncomfortable situations. If she were a raging bi#%$, it would almost be easier. I just know that I only get one life and I don't want to regret it. If I make a break, I risk regretting not making this marriage work, but if I don't, I know that I will regret not seizing an opportunity to live life like I want to. I feel so selfish, and I really try to be a nice person...this would hurt a lot of people and that just kills me. If I keep the status quo, the only one hurting will be me, and that is what a responsible person probably should do.

Well, I agree with 123...you need to have a long talk with your wife before you pursue this other relationship.  It sounds to me like you have some very serious communication problems in your relationship with your wife.  Have you discussed your feelings about your marriage with her?  If you do love her, are you willing to do what you need to do to make this marriage work?  Have you tried marriage counseling?  Before you start something when the other isn't completely finished is a recepie for disaster.  Likely, a lot of people, including yourself, will get hurt in the process.

If this other person cares about you, she'll understand if you at least try to make your marriage work, and if the marriage doesn't, then perhaps she'll be willing to wait for you.  I think you owe it to yourself to at least answer a few of the questions that have you confused before you go any further.  Just my two cents.  Good luck.

I have been down a very similar road: living more like roommates than spouses. No young kids, though (heaven forbid at my age :-). Going through the motions describes very much where we were.

My STBX is 100% convinced I was having an "affair" with a long-time female friend. An incorrect conviction, I hasten to add. Friend and I lost contact for about 1.5 years, but when things with STBX went to hell in a handbasket, I looked her up and we started having lunch together, talking mostly about what I could do to make life with STBX better. I noticed we did "connect" when we talked. And I was comfortable with her. Neither was true with STBX.

Not the same thing as being with a potential replacement for STBX, but I could do worse. What I realized, though, is that I couldn't go on the way things were. Neither of us was happy, and there was simply no joy. I chose to either make it work good enough to get some happiness, joy, and comfort, or split up. Since I call her my STBX, you can guess which way it went.

The real "bottom line", though, isn't that I left because I'd be happier with my friend. It's because I knew I'd be happier alone, just doing lunch or cocktails with female friends with no romantic interest, if that's the way life went. I knew I'd done what I could to make it work, but it didn't.

Before you consider splitting up, make sure you do the same. Be 100% certain you have done everything you can do and fixed everything about yourself that you can fix, before giving up. You do not want to look back years later and ask yourself "what if" questions.

My advice is to do the above; get counseling, ideally both of you, but for yourself alone if she's not willing. You need to know it can't be saved. And you can't get there if you're seeing the new girl.

Remember you probably felt about the same with your wife before you married as you do now with this someone else.