I need some advice regarding whether to sell my horse or not.?

Keep him. He's not going to "go to waste" while you're in school. He's just going to be a horse. And if you can continue to work with him on weekends you will still make progress...yes slower, but still progress. When I had an inexperienced horse that I could only see on weekends (due to a crazy work schedule), I hired someone at my barn to lounge him on Tuesdays and Thursdays for 15 minutes - just to keep her brain operating. They did voice commands and transitions, it was really good basic exercising You should be able to find someone that is good enough to lounge him once or twice a week, right? Plus it would be good for him to be learning to work with a handler other than you. Good Luck!