I didn’t get great grades in high school (or even technical college) either, but somehow I managed to do okay out in the work world as an adult. In fact, now and then I even had flashes of extreme competence. I still treasure the email I got from an engineer not long before I retired, complimenting me on the two huge sloping basalt-lined tailings lines that I had to model (in 3D CAD software) through a very messy and crowded old potash mill, out to a tailings pond. It fit together perfectly with no changes needed, which was a huge deal for those expensive specially-ordered fittings, and I kept the slope consistent the whole way.
When my mother cleared out her house prior to moving into care, she sent me a whole bunch of stuff including elementary school report cards. Over and over and over the same comments from different teachers – “R must try harder”, “R doesn’t pay attention”, “R needs to focus more”. It made me angry, because surely even in the early 60s teachers must have known about attention deficit disorder. Did they think it only affected boys? Didn’t they see that I was trying as hard as I could already?
No, school doesn’t actually have that much to do with how well you do out in the world. Focus on what you can learn from your classes, the knowledge you can pick up and how you might be able to use it out in the real world, and relax about the grades. I’m sorry your parents aren’t more supportive, but soon enough you’ll be an independent adult.
Take time out each day to relax and use your body doing something physical. It makes a big difference. I think I’d go nuts without my swimming.