Simple question, what are you getting out of this?
Even if you are hard to live with or crazy he has the option to leave as well. Marriages are partnerships. For either one of you if you feel the partnership is over then you have the right to end it. It's not a matter of right or wrong or anything like that. Its a partnership for mutual support. When the mutual support is no longer there what's the point of the partnership.
It's hard to work through I know. I am in the process of divorcing my AW of 36 years.
Don't get hung up on the fairy tale happily ever after and love is forever propaganda either. Everything changes. It's ok to feel sad but there is no need to feel guilty.
Your friend,