How do I (f20) prevent resurfacing trauma during sex with a new partner? (m22)

I (f20) recently (2mths) started dating this caring, funny and passionate man, (m22) and as things progressed I was excited to get into bed with him. I have had only one prior relationship about a year ago, with a man who was very manipulative, conniving, and sexually abusive. I took time to heal on my own, and have been transparent about my past with my current boyfriend, and he has been nothing but supportive.

I had been having sex a few times with my new bf without becoming triggered, so I figured I was in the clear. The other day, my bf mentioned how much he loves being topped, so I decided to go for it. But the combination of pressure from it being his favorite position, and flashbacks resurfacing, I ended up crying afterwards.

I told my boyfriend about it in full, and he was once again nothing but supportive, apologetic, but worried for my wellbeing.
I don't want this to continue happening, 1. because I want to enjoy sex without fear, and 2. because I hate expressing vulnerability around the subject, it makes me feel naive.

Advice would be greatly appreciated! Thanks in advance.

TLDR: How do I prevent resurfacing trauma during sex with a new partner?