I get what you are going through with your mom... when the people we love most don't believe in us it kills us and the fact that you have trouble letting it go means you do love her.
As for your boyfriend, if he isn't okay with you being an "emotional girlfriend" when you need to be he is not good enough for you. You need someone who is supportive of you and will listen when you are troubled.
Most of all, you are not an idiot, you do not sound like an idiot, you are just going through a tough time.
Most importantly, you can't let those grades slip. You need to get some kind of tutoring or something because you are right... art accademies don't just go on talent, they want a well-rounded student.
Most importantly DO NOT allow your boyfriend to be the "most important thing". He can rank up there, but school must be your #1 priority. If art and music are what you want to be a part of when you grow up, your education has to be your main focus.
Maybe this sounds cruel and you think I don't understand but I have been right in your shoes. I made a lot of the mistakes that it looks like you might be heading towards and ittook me years to get past them.
Be brave.
Hugs,
LizardL8y