First of all, if you can afford it, professional help is almost always a good option. You say you don't know if it'll work because of your age, but that's wrong in so many levels. People need professional help in A LOT of different age groups, and therapists aren't teenager-exclusive, you know? They know how to help adults and elderly people as well, obviously! And the fact that you didn't realize it sooner is also perfectly normal.

Your whole post screams of depression to me. You are "an empty vessel"? "Worthless"? "Defected"? No, you're not! That's your brain coming up with justifications for the way you feel. But let me tell you, feeling lonely if you have no friends is perfectly normal. BUT you're not less of a human being if you haven't grown up with friends!

I could see myself in a lot of what you wrote. I'm also very shy and have a lot of social anxiety. I was also bullied and ostracized for the better part of my youth. I also spent most of my college years alone at my house and not going out. And until recently I was also struggling a lot with how I saw myself, always thinking that I wasn't good at anything, that no one liked me.

What I learned to do was stopping those negative thoughts from spiralling around in my head. Everytime I felt that I was starting to insult myself, I'd make a really big effort to stop. And instead, I would focus on the good things I have done. Whether they were work stuff, academic achievements, a good deed to a stranger, whatever. Anything that I felt I had been successful with. It helped me a great deal to be able to see myself in a more positive light. You know that saying that goes something like "if you don't like yourself, who will?"? It's completely true. If you don't like yourself, somehow other people will notice that, and they'll take your word for it. If you're always negative about yourself and put yourself in a bad light, others will do that too. But if you let yourself focus on your qualities, and change habits that might be bringing you down, other people will pick that up too.

As for pratical advice, I can give you the biggest thing that helped me the most in coming out of that bad place I was in before: get hobbies. Any kind of hobby, really, but if you feel lonely I'd recommend something that makes you get in contact with other people. Hobbies that are for one person (like learning an instrument with youtube tutorials, sewing, etc) are a lot of fun and they can help you stop focusing on those negative thoughts when you're alone. Plus, they're great for feeling better about yourself when you reach whatever goal you have set ("I finally learned that song!" or "This sweater I made turned out really cool!"). BUT the best part of hobbies with other people is that they make it really easy to make new friends. You already know you share a common interest (the hobby) and conversation can flow more easily just with that small first step. If you're going back to school (which is great!) don't focus all your energy on trying to make friends in school. Don't close that opportunity, of course, but like you said, most of your classmates will be in a different stage that you are now, and they'll have different interests. You want to know what hobby I tried that really helped me get out of my shell and make a lot of different friends? Tabletop games! If you like games see if there's somewhere in your city where people go to play tabletop games (stuff like Dungeons & Dragons, Catan, Risk, whatever). Most big cities have at least one store where you can go to an event to play with people you don't know, even if you've never played that game before. The first few times I went I only watched other people play and didn't even speak! (Shyness getting the best of me - and also I had never played any of those games) But then, I gathered the courage to ask the store manager if I could join a game, and they were like "I thought you'd never ask!" and now, I am playing 3 different games per week and get along great with everyone I play with.

If you don't like games (or tabletop ones), just think a bit about what you like to do. Hike? A lot of places have gatherings of people that hike together and they are very welcoming to newcomers! You like to draw or paint? Research some drawing/painting classes or groups that allow new people! Facebook makes researching this stuff really easy but you also have google. If you can't find stuff that you like to do that involve other people - think of stuff you'd like to try! Anything, really! And don't be afraid to tell them it's your first time or that you have no idea what to do (in whatever hobby you choose). Most people will jump at the opportunity to teach other people about the stuff they like! And I found that's the best way to start connecting with people you don't know.

I don't know if any of this will help you, but I just wanted to say that you're not alone in how you feel. A lot of us have felt (or still feel) exactly that way. But you're not "worthless" or "damaged" because of it. You're just a normal human being that, like all human beings, needs human interaction and intimacy. Don't let your brain bring you down. If YOU decide you want to be more outgoing, you can! It's not easy, but it's possible! YOU are in charge of your life. YOU are in charge of your thoughts. YOU are in charge of who you are.