How to get over my insecurities?

I am currently in a friends with benefits situation. We have not done anything yet, but are planning to. The guy is a really skinny guy and I am on the bigger side, I don't think I'm fat, but compared to him I am a lot bigger. I am attracted to him, but it seems like my own insecurities turn me off. I know his type is skinny girls and he says he doesn't mind that I am a little bigger, but I just can't get over my insecurities. I don't want to engage in sexual activity if I am not into it because I mean one can only enjoy it if they feel sexy, but in this situation I can't stop feeling insecure. This has never been a problem for me before because the guys I have been with were bigger and liked thicc girls. I want to do stuff with him, but how do I stop feeling insecure?
How to get over my insecurities?
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