From what you wrote, it is indeed likely that you are suffering from OCD. The adults in your life may not understand how OCD works. It’s not a matter of just stopping yourself from doing the compulsions. You can’t. Doing them is what gives you relief from the thoughts. Those thoughts can be all-consuming.
School personnel sometimes don’t realize that the thoughts are so powerful they interfere with a student’s ability to concentrate and to do well. I imagine it’s hard to focus on a math lesson when all you can think about is whether you left the stove on at home, for example. Kids like you often start failing in school, not because they aren’t smart but because their thoughts are so dominated by their worries. Often they become isolated because their friends can’t understand what the big deal is.
Please take your letter and this response to your school counselor. Ask for help finding a therapist who is experienced with OCD. The good news is that it is treatable. There are medications that can bring the intensity of the thoughts down a few notches. There are things you can learn to do to manage the thoughts so they don’t interfere with your life so much. Many kids “outgrow” many elements of the OCD as their brain matures in their late teens and early 20s.
I wish you well.