Hello Doingmybest. Sorry you are struggling. As far as why you have this difficulty, I believe you answered your own question in the post. Remember that if your current boyfriend is the right person for you, he will be okay with you expressing yourself....including expressing problems or concerns. People cannot agree all of the time...if they appear to be then something is wrong....one or both aren't being honest about their feelings. Did you know that zero conflict in a relationship is actually one of the predictors of divorce?

You can avoid the snapping by slowly and gradually expressing yourself. Start small and simple. Does he always pick the restaurant or plan because you let him/just smile and agree? Next time, try saying where you really want to go. Before you are even in a situation of some sort of conflict or disagreement, tell your boyfriend that you have a new personal goal of working on expressing yourself more with everyone. Give him a heads up and ask for his support. He could even cue you in the beginning..."Do you have another suggestion to share?" etc

Your boyfriend does not really know you until he hears your true thoughts and feelings. Similarly, you don't really know his character until you see how he responds to conflict. So this is a really important area to work on.

It may be helpful to think about the scale of passive-proactive-aggressive.
People who struggle to express themselves often don't perceive a difference between proactive and aggressive but there's a big difference there. Also, you may like to think about the difference between disagreement and fighting.

Best wishes to you as you work on your new goal