Taken for Granted
Relationships take work, time, and effort. Sure, it’s natural for that initial chemistry to fade away after some time, but that doesn’t give you an excuse to stop trying. The OP’s ex-fiance clearly didn’t get the memo because at some point he just started taking her for granted.
He never wanted to do anything fun anymore or go anywhere and though she tried her best to fix things, he wasn’t interested. It’s easy to break up with someone who cheats on you or lies to you but it’s harder when you quietly fall out of love with someone because they stopped trying. While she still feels guilty for what she did, she doesn’t regret it at all!