Amy, to start with the ending: both of you took vows. He broke his a long time ago, so IMO the onus is removed from you.
The next thing I have to say is that life is supposed to be easier if you have a partner to share the burden. If it's not easier, there is something very, very seriously wrong. My ex was incredibly irresponsible, and I earned the money and did the cooking and cleaning and mothering for too long... it just about broke me. When I finally spoke out, he freaked out and decided he was leaving me. O, what relief! Life as a *single mother with two babies under two* was easier than living with him!
Sometimes, marriage is a toxic situation for one or both parties. Don't stay with him out of a sense of "should"; you only "should" if you, deep in your heart, feel the rightness of it. Listen to your heart on this, and your gut. Maybe a separation is what he needs to be able to grow up... and maybe he never will, but you don't have to be burdened with him forever.
After my husband left, life was HARD for a while... but still, I was happier than I was with him. And then things got easier, and better, and now they're wonderful. Incredible. My sister has been going through the same thing. If it's what feels right in your heart and gut, please don't be too scared or ashamed to leave; wonderful is in your future too.
-Kalera