Really, really struggling over my penis size, what do I do?

Throwaway account for obvious reasons.

I (23M) am… not well endowed. Fully erect, I’m just barely shy of 5 inches, and I’m unremarkable in the width department as well. Also, I have a normal BMI, so my size is not affected by any excess pubic fat. I have a partner, and I’ve been dating her for a year and a half, and she SEEMS to be satisfied with my abilities in bed, but I have trouble believing that…

My previous “relationship”, if you can call it that, ended when my now-ex finally expressed to me in a super delicate way that I am simply not big enough for her and that while she really liked me, she did not feel sexually satisfied and didn’t want to cheat on me. This absolutely shattered me; my self-confidence regarding my sexual ability has been at an all-time low and my current partner has absolutely noticed it.

I’d like to think I’m good at giving oral, and I’ve made her climax from my tongue alone quite a few times, but as far as penetrative sex, I feel like I’m doing something wrong. She can easily fit MUCH bigger and thicker, and has a few toys that are a lot more impressive than I am, to put it lightly. I’ve used those in bed as well with her, but it’s not a good feeling that I have to resort to that to please the woman I love.

I don’t know what to do. I have absolutely ZERO confidence with my sexual abilities anymore and there are times I feel like breaking down in tears. But I don’t show it and try to be strong for her. She says she’s very happy with my sexual ability but I just CANNOT under **ANY** circumstances believe her. Not one bit. And I want to. I really, really want to believe I please her. I just don’t know what to do. I really would appreciate any advice, any at all.

Thank you.