Being an only child is no fun as I was one. Although my parents were not that hovering about homework, they were crazy about other things. That was a long time ago, and now there are social workers, good school counselors etc that can intevene. In my day, you didn't talk about problems outside the home. Now, you can and you should get someone to listen to you. First though start with mom on a good day for both of you..and if things are difficult and she does not understand..then....Try your high school, a trusted teacher, a friends mom, a minister, a youth minister, or if you are in therapy, the therapist...I almost wish you were in therapy as it would then be super easy to have a good discussion with a truly objective person.
Also you can ask if mom would take you to a therapist but that would probably cause a lot of difficulties. You did not mention a Dad or grandparents? If they are around maybe you can have a confidential talk with them about mom's behavior.

I vowed that I would never have myself an only child just for those reasons, and all the problems I went thru...anyway I have three grown children now. I hope things work out. Please talk to a real person about this. I see its upsetting you, and I know its about mom's expectations..
.Take care of yourself!
Hugs;
Dee

p.s. the trouble with being an only child too is that mom has all her eggs in one basket. You are her only kid, and all her fears and worries and expectations go directly to you. She might not realize what kind of pressure she is putting you under.....so as I said above, sit down with mom over dinner, a good time for both of you, and tell her how her hovering and concerns are making you feel!!