How to be less conflict avoidant?

(Probably) due to my upbringing, I am HIGHLY conflict avoidant, to the point of detriment. I’ve never so much as fought or argued with a partner, even in situations of abuse. The one time I had a disagreement with a healthy partner, I got dizzy and lightheaded and felt like I was going to pass out. I never stick up for myself and feel like I’m never in the right. I DO get angry sometimes, but I keep it inside, even if I’m being wronged. I feel like one day I might explode and I don’t want to ever do that. How can I manage conflict in a healthy way?