Well, here we go again.
Your title says it all. You are completely stuck in a mental rut. You act like it's outside of your control.
You cannot control how people react or respond to you, because that would be manipulative. But you can control your mental and emotional response. Yet you keep looping the same "Well, here we go again" cycle in your head, over and over again.
That's why people keep suggesting therapy, because no one online is equipped to reach out through the internet and give you a shake.
You need some kind of epiphany. Meditation or some kind of zen buddhism "live in the now, the past is the past" medicine (literal or figurative).
Is life supposed to be this hard? Should I just stop?
You're asking these questions because there's a part of you, deep down, that wants to hear "No, don't stop, don't quit. It gets better."
No. Don't stop, don't quit. It gets better.
But it's going to take a change, possibly doing something you don't want to do, like seek out affordable mental health counseling. Showing your face at the doctor's, even when you would really rather not.
Just please tell me what I'm doing wrong!
You assume that there's an easy answer, a quick fix. But sometimes the best solution is the one that we work for.
You're like an obese person going on and on about how impossible it is to lose weight because you refuse to change your dietary habits and join a gym.
You're like a smoker who doesn't believe that quitting is possible, because all the quitting cessation programs/suggestions/advice all take you out of your routine and make you do something you are afraid you cannot do: quit the bad habit.
Getting therapy and finding out if medication is right for you is the equivalent of joining a gym and making healthy food choices, every day, day after day after day.
People can change. People can make better choices. It can be a struggle, but it gets better.
Maybe you need a complete change of scenery, like volunteering in a third-world country (habitat for humanity? peace corps? non-profit aid organizations?) so that you can have something to focus on besides going around the same mental loop over and over. Maybe something like that would shake you up and get you out of your rut (while also doing some good in the world, which is honestly a good thing for your own mental health, from my personal experience).