I'm not sure what to do

For the last 5 years at least, I've been absolutely terrified of going anywhere alone. I always ask someone to go with me. If I can't find anyone to go with me, I usually try and find a way to avoid going anywhere unless it's completely necessary that I go. When I do end up in public alone, I don't talk to anyone, keep my eyes to the ground, and it always feels like someone is following me or is going to do something to me. I'm totally fine around my friends and my family, but put me in a social situation where I don't know anyone and I shut down and find it difficult to start any conversation myself.. My body feels very tense and I usually stare out into space until someone starts talking to me first. At work, I manage, because talking to the customers is part of my job and I have to do it, even though I don't like it.
Another thing that is constantly terrifying me is thoughts (that appear at what I find to be random moments) that I'm crazy and will end up in a mental hospital. I don't even know why this scares me so much, because there's nothing that I can tell that's triggering this.
I see a psychiatrist for other things, but I'm scared to tell even him about these fears. How can I manage to work up the courage to do so without backing out last minute?

[JGF25]

Trust me when I say that these psychiatrists, they have heard it all and then some.  You could not possibly tell him anything he hasn't already heard before.  You are paying him good money so get in there and get your money's worth.  Tell him everything that is bothering you from the thoughts (and nothing is too stupid or irrational to say)  to the social anxiety because in the end he can't help you fix what he doesn't know is broke.  Do you really want to go on living your life this way?  Get it out and soon so you can start working on the solution to this problem.  My best.