How to gain a healthy amount of aggression

I have never dealt with conflict well at all. My body goes into extreme fight or flight mode and normally I flight. I have no fight in me whatsoever. I am very passive. My husband is not. It’s seeming to pose an issue in our relationship. I do not support him enough or give feedback when asked because I fear saying the wrong thing. And I am like this with everyone not just him. I can never defend myself and would probably curl up in a fight if it came to it. I don’t want to be this way anymore. One for fear I will lose my husband. And two I don’t enjoy being passive and never being able to say what I think or feel.

Does anyone have any advice or tips to gain a healthy amount of aggression or oomph?! My whole being is very lack luster (anxiety and depression play a role). Anything would help. Thanks