You need to learn how. You'll never figure it out, just sitting around thinking about it. I have a similar tendency to withdraw, and I force myself to engage with people more than I'm really comfortable doing.

The main thing to remember is that you don't know people, until you know them, and that takes time. In the early stages of being around anyone, admit to yourself that you really don't know what they are about deep down inside. Some of the worst creeps can seem likeable and just fine, until you have been dealing with them for awhile. And vice versa. Some people who don't seem very impressive, initially, can wind up showing themselves to be caring and dependable. Plus, once in a while, rotten people do some nice things. And good people, occasionally, aren't at their best. Don't come to big conclusions about people based on a little knowlege about them.

The main thing I've learned is don't think you know someone, until you do. That usually takes longer than we expect it to.

In your case, you know your parents and they treated you pretty bad. That's who they are . . . and they are not likely to change. A person's basic nature forms early in life and, once it forms, it's not likely to change a whole lot.

I read in threads about how people have been badly hurt by someone who was really mean to them, and they're hoping for an apology and for "healing" and a chance to have a good relationship in the future with the person. What a waste of time, IMHO. Time to cut your losses and move on. I'ld say: Be pleasant as you can be to everybody, but trust no one who hasn't given you a solid reason to invest your trust.