I wonder what you would think of the concept of an anxiety ladder/fear ladder. Perhaps you know of it? The idea is to train yourself to overcome your fears through gradual exposure to them.

For you, this concept might mean something like the following. You identify (say) 10 reasonable, fear-related things that you would like to be able to do. Then you rank them from most scary to least scary. Then you start with the least scary. You start systematically bringing yourself to do what is the least scary. (If you find it impossible to do that, then identify something even less scary). You have a program to do it every day, or every other day, whatever seems best. If T does any CBT (s)he could help you with this. You gradually over come that first fear, and then move up the ladder to scarier tasks.

Another idea is to look very closely, step by step by step, at a few incidents, one at a time, in which you wanted to be more assertive but you were not. You identify the point at which it was too late to change the outcome, and you look for the instants before that time where you could have done something differently. What could you have done and what prevented you?

With all of these situations where you are not assertive enough, remember that it took a time for you to learn to be in assertive, and it will take time to be to learn to be assertive. Keep working on it regardless of what happened. If you fall short, look carefully at what went wrong, acknowledge that these things are happening now because of what your childhood was like, forgive yourself for any shortcomings you sense, give yourself credit for working on change, and then move on.