How to discuss your feelings

How do you discuss your feelings and what bothers you to your partner if you have a hard time expressing yourself?

I’m introverted and I’ve never really had to tell people what I feel because I kept to myself.

Now my boyfriend gets upset at times when I can’t tell him why I’m sad or upset ( especially when it has to do with him ) bc I’m so used to dealing with it myself and not bothering others.

I’m trying to be open more but I still get anxiety every time I try, I just want to see what can help me overcome that so I won’t have such a hard time voicing my thoughts.