First, you should seek professional help. The sooner, the better. You're not alone. I know exactly what it's like to sit there in a classroom while a professor drones on about a ridiculously complex subject and then hating myself for not immediately understanding like some kind of brilliant super genius. It's irrational, and it is treatable.

You should speak to a therapist, maybe get some medication.

No offense, but you also don't sound like you're a very social person. Education is important, but in my opinion, socialization and communications skills are even more important.

It's okay to be an introvert. It's not okay to be incredibly awkward and uncomfortable around people, and therefore either isolating yourself or driving everyone away. Again, if this doesn't describe you, I apologize. It's hard to be happy when you're totally alone and disconnected. Also, an advanced degree is a great thing to have, but if you can't communicate worth shit the only thing that degree is going to do for you is open the door. You aren't going to go far on it alone. There are countless engineers out there working for Lit majors who can agree to that. If you want to be wildly successful you need to find a way to get the advanced degree without sacrificing your ability to communicate and make common sense judgement calls.

You're approaching communication like an engineer. You're analyzing it. You're trying to work it out in some neat, clean way. Communication is an art, not a science. It takes practice to get the hang of it, but the first step is to let go. You need to work on your confidence so that you're comfortable with that. Don't think. Don't analyze. Just be in the moment. Let the words come. Those thoughtful, clever people you talk to aren't carefully thinking of their next words like they're writing a book. They're caught in the flow. They're enjoying themselves. They're responding with emotion and gut instinct. And they're confident enough in themselves that they fluently tell jokes and laugh and smile, and if a joke bombs that's even funnier because they roll with the punches and laugh at themselves right along with everyone else.

You can be a sociable guy. You can also be academically successful. Takes work and practice, though. Get into therapy. There's no need to go it alone. All the best!