Hello G: I'm sorry I don't think there is really much of anything I can suggest with regard to this. (Perhaps other members, here on PC, will have some perspectives they can share.) To me, this seems like a concern that is perhaps best delved into, at-length & in-depth, with the help of a skilled mental health therapist. Here are links to 5 articles, from PsychCentral's archives, that may be of some interest, the first by our host Dr. John Grohol, Psy.D.:

10 Reasons You Can't Say How You Feel

Building Assertiveness in 4 Steps

5 Tips to Increase Your Assertiveness

Inner Strength: How to Build a Firm Foundation for Living Your Best Life

How to Build Self-Confidence, Inner Strength, & Resilience

I hope you find PC to be of benefit.