BigClog, against all the good advice you are getting here I'm going to say something a bit controversial: Not all marriages deserve to be saved.
A year ago, I was were you are, feeling like you are feeling at the moment, although our reasons and circumstances were totally different.
We had worked for years trying to get the marriage back in good shape, we were bickering to each other, the attraction was totally gone, I started to explode for even the more insignificant things, every intromission of my MIL sent us into days of resentment, and recurrent depression was part of the last few years, and at some point I started to "realise" the grass was greener.
One day after a most stupid discussion, exH said he didn't love me and didn't want to be with me anymore, this came a bit as a surprise as I had spent MONTHS trying to find a way to tell him, without hurting him too badly, that I wanted out. We both had been there for the sake of DS for a long time.
What followed was quite a surprise, once the initial shock and worries of the separation were gone (or eased!) things are FANTASTIC. I'm so very happy to see him going back to be the person he once was, I have had a fantastic time myself, these last months have been the best of my life. DS is also well, benefitting from the extra attention he gets with the time with each of us, receiving our sole attnetion now that the time we have him with us has became precious.
DExH is again one of my best friends, we go out as friends with other friends without a problem, even have managed a holiday together where we all had a great time. Mind you, no attraction whatsoever left, no intention to go back at all from me or him.
What I realise now is that all the bad times we were giving each other and the sensitivity to certain circumstances of the past were not causes for the attraction to disapear but symptoms that it was gone. We had stopped loving each other and although there was always great respect for each other, which moved every single marriage counselor to insist in us trying it again, which made our friends thought we were the "ideal" couple, was not actually love but a friendship.
People find it difficult to understand how you can not continue married to a friend once the atraction is gone. Every time somebody mentions that, I ask them to think about that lovable guy who wanted to date them, he who was so perfect that they would have married him to their sister or their best friend but despite all efforts they couldn't bring themselves to love. Well, it is very much the same. I have tried to love him again, and I have tried bloody hard. I have been VERY ANNOYED with myself at not being able to do it, because we are very compatible in many aspects, we had a good life, but sometimes things are not meant to be.
So... better to leave it if you think there's no way back. Stop wondering about what could have been different or the past. If you don't love him anymore or have not love him for ages, the chances for this love to return are near to nil. We realised at some point that it was better to leave it while we could still talk to each other than wait until our frustrations piled up to the point of getting into those very bad things that some times happen at the end of a marriage.
Good luck.