Money problems can be very hard :( And I know people on this forum can say many things, and while everyone means well hearing all this stuff can be pretty hard. In the "real world" you wouldn't have a group of people spitting blunt and honest advice all at the same time. Its not a very natural way to learn.
Horses are a very expensive hobby. That's why when you tell people you have horses they say you are lucky, why not everyone has them because they are both time expensive and financially expensive and not everyone can afford them.
You can get by on them "cheaper" sometimes, by getting friends to help out, like you do with the farrier, or working off board or buying feed from farmers and just general things like that. And that is really good but these are factors that while may help, you should never rely on them, or go into horses relying on them. A vet bill for an injured horse can go into the thousands, so there should always been that extra room in your income for extra costs. Things like having a dentist and a farrier out are not little luxuries, it needs to be done regularly whether you ride your horse or not.
With the other things, you could always not ride for a while and put them off. But most people wouldn't do that. My horse once had a bad back that had been caused by years of bad saddles, and then exasperated by a new farrier who didn't do it so good. So I got someone to work with his back out a few times, and a really good, but expensive, farrier and I gave him three months off riding. I worked with him doing exercises on the ground, getting more flexible, stretches. When he came back to work he was better than ever, I got the saddle refitted to his new condition and since then, until I sold him, he was great. Not all people would do that, it really depends what you put first with horses.
Beyond the financial problems, you're not enjoying riding anymore and a big part of this could be an inappropriate horse.
I actually do understand what you mean about your mother. Mine was a single mother and she would pay for the horse, and come out everyday, and whenever we didn't have money or time I felt as if she blamed me. That it was all my fault because I was selfish to want and love my horse. It was beyond that, I always felt like I had stopped her living a life she had wanted to live, that she had to care for me so she couldn't go traveling, and live by the beach and have a career. For the longest time I felt that I had done this to her, and in many ways I still feel that. I told her once and she was very surprised and said something along the lines of that is just what parents do, and she had wanted a child and that was a life she had chosen, and she hadn't had a problem with it. I guess I am understanding more now, as I grow up and understand priorities in life. And while you may want many things that are often mutually exclusive, some things you want more, and choose the paths you take. Like I want to have a horse and I want to travel and I want both of those things a lot. But I chose to travel this year and while I miss horse riding and would like to do it, I don't regret my decision and am happy I traveled. I can have horses all my life but I won't always have the opportunity to study and live overseas. She made her decisions, you shouldn't guilt for your parents.
Your mother chose to let you get this horse, and she is choosing to keep it. If it really bothered her she'd tell you that you guys couldn't keep her anymore.
To be honest, with all these problems added together, if I were you I would consider perhaps selling, trying to find a good home that would work with her problems, and maybe taking lessons for a while. Get attached to a local riding school where you can help out on weekends or something, volunteer for horse use. The best thing being that there are no surprise costs and you won't be committed to anything. And maybe you can have fun riding again. Its not forever, you can always get back into them again, buy another one.