Girlfriend (20F) cucked me (19M) and I’m so overwhelmed

Hey everyone I’m new here. Sorry if this is too long. Just need some advice about everything tbh.

So basically I’m 19 (male) in college and my girlfriend is 20 and goes to a separate college. We have been dating for 2 years now and I love her more than anything. Our relationship has been so perfect and we both deeply love each other. I’m much more of a romantic in general and I’m much better at all the cute /sweet stuff than the sexy stuff (if that makes sense?). My girlfriend has always told me how much she adores me and how lovely I am etc. Unfortunately, the massive elephant in the room is the sex life. I can’t lie I’m pretty poor in bed…and she knows it too. Unfortunately I wasn’t blessed ‘down there’ and I’m only 3 inches maybe 3.5 fully erect. My girlfriend is really pretty and has had a lot of casual sex before our relationship and it really hurts me inside knowing I’m unable to satisfy her.