How to tell him i'm unhappy 

I'm so where you are. I also have three kids, the eldest being my daughter from a previous relationship. I told him last week (or began to tell him) how I was feeling. He took it too well and I realized it was because he hadn't really taken it seriously. A week later I told him again and it was a huge shock. Lots of crying and I just stood there and watched him, I didn't know what to do because I was numb, I didn't have any feeling that I could respond to. Surely if I still loved him, my immediate response would have been to fling my arms around him and tell him it would all be ok. You feel guilty, just like I do and the lack of justification for falling out of love with him scares you and makes you feel like you're the biggest bitch of the century. Well, I don't think we are. Whatever the reasons are for feeling the way we do, we have to act on them, we can't just keep bimbling along in loveless marriages hoping that one day the spark will come back. I'm finding it very difficult to separate sentiment and real feeling right now. I'm grieving for the feelings that I had and that are no longer there. If this guy wells up when you speak to him, it means he already knows the truth. And just like my husband he wants to stick his head in the sand and hope that it will all go away. You're sad, guilty and confused and I'm totally there with you girl. Good luck to us eh?

I agree 100% with mtnvly...I highly suggest the two of you get into marriage counseling - both of you are unhappy and hurting...

Sometimes when we our relationship lasts as long as yours has the spark of romance tends to disappear. We stop sending flowers and little love notes . At this point you need to work harder at it. It is hard to "romance " someone you have been with for so long. I would say you are still in love with him because I don't think we "fall out of love".
Try something for me. Start calling him thru the day to see how his day is going. Stick little notes where only he can find them .Plan a weekend away somewhere for just you and him, no kids. Make sure you are making time for making love. It is an important thing for couples. Just try throwing a little spark and see what happens. Some counseling may help also , for a 3rd party to hear you talk and give advise.
If the above does not work then think about separating. But please try it first.
Marriage is tough. It has it's ups and downs. But the grass is not always greener on the other side. So please don't throw it away without trying all that you can.