I am going through a break-up with my (x)SO as well, although we have been together much less time than you have. It's so hard to walk away, but it's like investing in a stock that is going south, you need decide when to just take your losses. No point in continuing to throw good money after bad again and again.
It's sooo hard, but we can't fix people, as much as we want to (I've heard this a lot in the past week myself!)... it is up to them. The only thing we can do is figure out if we can live with them "as-is" or not. But, it's our choice. We get to make the call.
Good luck, I hope it works out for you.