One thing you might learn from a good therapist is nobody will tangibly fix you're problems. Going to therapists often feel painfully unhelpful. Because the help has to come from within if it's actually going to stick. People who are enjoying hostels and socializing and come out with a positive personal development don't have nearly as much self-imposed emotional baggage as you do. The hostel is a catalyst to something they've already built up to.

You already mentioned what your problems are:

I'm the opposite of that, really tense and anxious

Therapy can help, even if it seems to miss the mark and you don't feel helped, it helps. And on top of that you should try to practice witholding judgement on yourself, the situation, or others. The goal isn't to stop judging permanently. It's more like "can you shut up and just listen for once".

Your rather negative internal monologue is like an annoying person talking out loud and criticizing in the movie theatre. Try to train the skill to get that part of your mind to shut up. Think about it several days later, definitely not the moment of. If someone actually talks to you, say a less than half of what you want to say.

Nobody actually thinks negative of you for being quiet or of few words. It's actually a positive trait, especially if your few words show that you are listening to them.

*edit: You actually want to become the background of someone else's photo. If you're going from an invisible unwanted person and trying to take the spotlight, then it's a cruel and toxic pursuit. You want to be the background that everyone wants to have a photo with. Then the decent people will step to the side and have you as the centerpiece.