Sorry to hear what you're going through.
It's a lot harder when there are kids involved, but you can forget about the sunk costs. That's not a reason to stay in anything...it's all something that happened in the past. You need to look to the future, because that's what awaits you.
It sounds like you've made a lot of effort to get it to work. My ex and I had two years of couples counseling once a week, and while it helped us to understand each other better it didn't resolve the underlying issues, which were largely due to deep personality differences. We were great housemates, just not great soulmates, and while many of my other married friends felt that it's a lot more important to be able to live with someone than to be in love with them, I just felt alone. Everyone's situation is different...you can try to get back to the beginning and feel again all the reasons why you fell in love with someone, but often there's too much water under the bridge, too many debits to your emotional bank account, to be able to get back to that starting point again.
It sounds like you're ready to leave. Your baby is young enough that a divorce probably wouldn't be nearly as traumatic for the child as it would if she were older. Still, if you think your marriage is worth salvaging then it's worth sticking it out to see if you can make it work. Friends of mine were on the brink of divorce and decided to stay together for the kids' sake, basically just co-habitating and living their separate lives. But that decision seemed to liberate them somehow and they could be honest with each other in ways they couldn't before, and now they're doing well. I don't know if it'll last, I have my doubts, but it's one of the few hopeful stories I know of where a couple came back from the brink and became stronger. I think it depends on whether you think there's hope or if it's a lost cause. There's no magic way to know, you just have to listen to your gut and get perspective from friends and other outsiders who aren't actually in your situation but observing it (which often gives them better insight since your judgement may be clouded).