My mare had to be suddenly put to rest between by senior year and first year of college. I left for college, and rode my other horse once per break. It wasn't until two years after my mare had been put to sleep when I started asking when I could move my other horse near my college with me, and within a month, the move was made. Now, I ride two to three times a week, train horses for others, and I even bought my own training project.
Riding is supposed to be fun, and if it isn't fun or motivating right now, give it a bit of a break.
If you miss it and want to, quit being whiny about not being motivated and get out to the barn. I'm guessing you stay up late finishing your homework and spending time on extracurriculars/friends, but get out to the barn on the weekends. Don't give yourself time to find a reason not to go, JUST GO!
Schedule a weekend lesson, so you can't miss it. Find a new goal you want to reach with your horse and work to reach it. If you have any interest in competing, plan a weekend that you are showing and use all the other weekends leading up to it to practice and prepare. If you have made any new friends at school, invite them out to meet your horse and go for a short ride. If you have any friends with horses, plan a trail ride or some arena games to do on a weekend day.
Toofine - 1998 Half Arabian
Minnie - 2013 Morgan