In my case it's my fear of being abused or taken advantage of or simply invalidated, but I think in both cases it leads to same thing: to fear vulnerability and avoid it.

For me, the way it works is to go slowly and slowly build trust. However, sometimes I may, say, share too much personal stuff and feel real vulnerable as a result. Even on this forum, for instance, then next day feel terrible pain and regret about sharing what I did, especially if did not get as many responses or felt someone was kind of mean or the responses were not caring enough. Then I think, I WILL NEVER EVER will share personal things here!

This, however, usually happens when I overshare. And why do I overshare? Because I stop myself from sharing even a little. So it just spills out. So I've tried to share a little at a time. That usually works better and I feel more comfortable that way.

Perhaps it can be helpful to you too. That way, you can keep checking if abandonment happened. You will see, I'm sure, in majority of cases it won't. Then you will see that you have exaggerated particular instances in your mind, when in reality it would be rational to expect more positive than negative experiences, after showing vulnerability sometimes. Then it will become easier and easier to show vulnerability. Good luck.