krib, for what it's worth, I think that you need to concentrate on why you are feeling this way. I would recommend going to some councelling on your own. Not neccessarily to talk about your marriage, but to talk about yourself and what makes you feel happy and fulfilled. It is very difficult when you have been in a relationship for a very long time to imagine how things might be if you were not (especially if like me you started your relationship young, and haven't ever been truly on your own). Also whatever decision you make you are not in a good place right now, and from what you have said, if you did walk out it would be very difficult. So you need to make sure you are coming from a strong place.
The one thing I would say to you is that you should never try the line that by leaving you will help your dh. My dh tried that when he was having his affair, and it was a completely hurtful selfish thing to say. It may be that in the long term it is the right thing for both of you. It may be that you will both find someone new, that your children will be happy and that it will all work out fine. Or it may be that you, your children and your dh will all feel sad about this for many many years to come. Or a mixture of the two. The trouble is that there is no way you can tell. You are not entirely responsible for your dh, and you need to stop feeling "over responsible" and motherly to him (it's probably not helpful regardless of whether you go or stay, as it's not a very adult to adult way to interact, although it is very common!). On the other hand you need to treat him fairly, and not lead him on. And you need to be true to yourself too.
What you mustn't do is leave half heartedly. You need to start to think about things very realistically, and give yourself a plan. So think things through. Try councelling to make yourself strong, and to be really sure about whatever decision you take. Think about the logistics, where would you live, how would childcare work out, what financial arrangements can you make etc. Once you have thought things through, and have some solutions, then talk to your dh. You may find Relate will help you to do this talking (they do provide sessions for those seeking to break up "well").
Good luck, and make the right decision. Only you will know what that decison is, but try to make absolutely sure that it will be one you can live with (you may find it helps to think about how you might talk it through with your children when they are grown up - ie in 10/15 yrs time).