There's no time like the present. Break up with her today. In as diplomatic a way as you possibly can.
Or tell her that the exclusivity is over. Which is pretty much the same as breaking up with her.
For your next relationship, lay off the trying to change the other person. If they want to change something about themselves fr the better then by all means support them in that. But otherwise accept them as they are. Nobody's perfect.
Also, next time, try to go on more mini-adventures as you hit the 2nd year of the relationship. The longer you're with someone, the more important it is that you do the sort of stuff that married men do with their secret girlfriends.
I don't blame you one bit for losing your attraction to her. If I were in your shoes I'd probably be thinking "Sweet girl. But not for me."
It's always a huge mistake to stay with someone because you'd feel guilty about hurting them.
One way to look a it is that you've had 370 wonderful days getting to know each other and then being sexual partners. And now she's going to have 3 to 37 bad days when you split up. That means that 90% to 99% of her days will have been good ones.