I really feel for you. You're taking all the right steps and doing your best to continue on despite the hardship you're facing.

I'm recently faced a somewhat similar situation, but with a family member. There are several registered sex offenders that live in my community. One has made a habit of watching several kids very closely, including my brother: he'll stalk them, waiting in his car just outside of school boundaries, near their hangouts, bus stops, etc. The cops can't do much except talk to the guy to let him know that he's being watched. When the kids began to notice him and found out who he was, they began talking about his crimes. My brother(10yo) wouldn't even play outside alone or walk down the street for fear of being kidnapped. It was so crippling that for a while he became mildly agoraphobic: refusing to go outside or to open places at all.

The best advice I could give him was that there are, in fact, nasty people out there: people that will rape children, or shoot complete strangers for no reason at all. Part of having freedom is understanding that you can't allow people to bully you into being afraid to live life. Rights come with responsibilities: if you're going to allow others to walk all over you, you're passively giving up your right to exist freely. In the end, my brother got over his fears one day at at time, gradually opening up his willingness to be outside again: learning to avoid undesirable people and situations by taking simple precautions. As a matter of fact, the creep is still watching my brother and his friends, but he grew to not let it affect his life.

It might not be a direct correlation, but your situation is sort of similar. The man intimidating you is a numbskull that thinks it's amusing to pick on a young girl that handles packages. He also sends his son over to screw around with you since he was banned. He's a particularly low person with little to no care for the harm that he brings on others with his reckless behavior. Will he actually shoot you? Maybe, maybe not. You state that the chances are low. There are others like him out there: maybe even living near you. Why would I tell you that? To worsen your condition? Not at all: my point is that there's no end to the crazies and weirdos out there. Whether they're ogling preteens or harassing young women, they're here to stay.

Your job, nay, your responsibility as a person living in a free society is to learn how to live free from intimidation. Do whatever it takes. Firstly, continue therapy vigorously (if you don't see results, change therapists). Also, if the son/father intimidate you in any more creative ways, file a police report. If they continue to harass you even further, take legal action. (xpost in r/legaladvice just to throw some ideas around) Also, it can't hurt to build your self confidence by learning how to shoot, taking self defense classes, etc. But most importantly, do the basics. Keep an eye out for who's coming in and out of your workplace as well as their behavior. Don't go out alone at odd hours of the night. Keep some mace with you. Park in well lit areas. There are tons of people that get caught up in horrible crimes, but there are many more crimes that are prevented by astute, self aware people that don't put themselves in bad situations.

If you quit this job out of fear, he does indeed "win". But a far more important thing to consider is: how many more crazies will you meet at your new job? And how many more jobs will you have to quit out of fear?

In the end, like many out there, you have to decide what's more important to you: living in fear or living free.

You can't do both.