This sounds like a really unpleasant and frustrating situation, and I'm sorry you're dealing with it. The first thing I'm going to suggest is that you two not engage in anything sexual unless you are both eager and happy to do so. Right now it sounds like you're being sexual with her out of a feeling of obligation, which is a crummy reason to be sexual with someone. So, taking sex off the table for awhile might be a good call. Do you feel like that's a boundary you can set with her?
Beyond that, it sounds like the two of you need to have some conversations about how to make sex pleasurable for both of you. The tools in this article can help you with that process: Be a Blabbermouth! The Whats, Whys and Hows of Talking About Sex With a Partner
. As an example, you may want to talk about spending more of the time your sexual together on things like oral or manual sex, or introducing sex toys into the mix. A big reason for that is that vaginal intercourse alone isn't something most people with vaginas find helps them orgasm or gives them as much pleasure as other activities does (and that's regardless of the size of their partner's penis). You can read more about that here: With Pleasure: A View of Whole Sexual Anatomy for Every Body
. So if it's not pleasurable for her and it's not really doing it for you, there's no reason to make it the focus of your sex life. Does that all make sense?
Too, in case her comments have left you feelings pretty down about your penis size, you may want to read this article, as it covers a lot of myths about penis size (including what counts as "average") Shown Actual Size: A Penis Shape & Size Lowdown