You and I are similar ages, so maybe my experiences can be relevant.
I just realized how long this was, so cliffnotes: Take chances, things will improve, and once you get settled into your new gig, you'll probably be much happier.
I was in a similar rut in 2009. I was working part time, I was bumbling around at school, and I was rarely happy. Running became my distraction, maybe even my obsession, and I ignored most everything else that was going on, simply because I was not doing anything well. Around November of that year, I decided to make some drastic changes so that I could hopefully pull myself together.
My first step was deciding not to re-enroll for the spring semester. I was in college out of obligation, not because I wanted to be there. I was trying to swim upstream with that attitude, and I realized that I needed to take a break and decide if higher education was something that really had value for my own situation. I started bugging management at about a full-time position, and after a month or so, I got a full-time, entry-level management position. When I got home, I was tempted to sit on my butt and watch Law and Order all day, but I did everything I could to better myself at work and outside of work. I read, I cooked, and I saved money. Only having 40 hours of work each week and nothing else leave a lot of time for oneself; I decided to make the most of it.
After 4-5 months, I decided that I was ready to return to school. However, around the same time, one of the managers really wanted me to take on another position at work with a lot more responsibility, but not much more pay. It wasn't a position that I necessarily wanted, but I was able to negotiate one crucial element into the agreement: I could work full-time around my part-time school schedule. So, I began the new position, and a month later, returned to school to follow my own pursuits.
This past summer, after being back in school for a year and doing pretty well in the position at work, I was offered another position at the other location in town. This paid much better, and it was one of my realistic ideal jobs. Thankfully, I once again negotiated working around my school schedule as one of the terms of employment, and I continue to juggle both.
What I am trying to say is that you should swallow your anxiety and go forward with this new position. Show that you are in it for the employer (always put your success in terms of the employer's success), and try to think long-term. I'll close with some advice that my dad has given me in times of anxiety: As long as you are young and have no dependents, you are free. Take chances now while they are still easy to take. When you have a family in tow, you can no longer live solely for yourself.