I want to split up with my girlfriend because my physical attraction to her has diminished. I know I'm not the best looking guy either and while I think she's perfect for me in a lot of ways, I just find myself less attracted to her these days.
So are you going to tell her that you don't find her attractive anymore?
Yeah I might say that because it's how I feel.
While looks are important, there are more things to a relationship than just physical attraction. You should think carefully about if you truly value looks over those other things you mentioned before.
I might say I want to date younger girls
Although these are fairly superficial reasons to break up with someone, if that's how you really feel you should be honest. It's better to end the relationship now than to lead her on and waste her time.