How do I make myself comfortable with sex when I’ve been through traima

I’m 19 years old and I’m in a new relationship.

Unfortuently sex is hard for me, especially certain things during sex I just get flashbacks and get emotional.

My boyfriend was trying to make me bend over but I just got emotional and I told him to stop he was just telling me it’s okay it’s minor (means fine) and it’s calm I’ll like it but

I just couldn’t do it

Like I just started tearing up

He then told me he’s never having sex with me again and just went sleep and became grumpy with Me

I just want to force myself to feel comfortable I don’t want to lose him. What should I do.

He knows about my childhood and I explained it but I just feel so bad because he really wanted to do it.