That stinks your relationship has continued to be so up and down. We know you care about her a whole lot, but the stress and emotional roller coaster just aren't healthy for you. Have you spoken with her about how she has been making you feel? Of course you have to be there for somebody during their ups and downs, but if she is consistently treating you poorly then that is a problem which needs to be addressed.
Just calmly and politely let her know that when she does specific things it causes you to feel specific ways. Obviously being specific is key and much more helpful than simply saying, "You make me feel bad sometimes."
Whenever you put so much focus into one area of your life (the relationship with your girlfriend), it can be very easy to feel overwhelmed and down on yourself when things start to go south in that area. In order to counter this, it helps to set goals and have high expectations in other areas of your life as well. Setting and reaching goals really does help give you feelings of purpose, direction, and accomplishment everybody benefits from.
Start by setting short term goals, things you can accomplish within a few days or a week. Instead of putting so much emphasis on the relationship with your girlfriend, allow yourself to feel the fulfillment in other areas as well.
A lot of parents put pressure on their kids to do well academically. In order to show them how much you are trying, it helps to be open and transparent about your academic pursuits. For example, when you get home at night, show them empty worksheets or assignments that you need to complete, then go do them. After you have finished, take the completed assignments back to your parents to show them how much you've accomplished.
By polite and pleasant when you do this, though. Don't be all, "Hey...SEE I got my homework done!" Instead, be polite, "Hey mom, here are those assignments I showed you earlier. I got them done and think I did a pretty good job on them." Also, talk to some of your teachers and ask for help before or after school. This shows your parents that you are taking the initiative and extra steps necessary to put forth your best effort. Doing that a couple of times could go a long way towards winning your parents over.
Being a teenager is tough, but keep hanging in there. You've got a couple of issues in front of you, but there are definitely steps you can take to get through this and find a positive outcome for yourself. A person's perspective can make an absolutely huge impact on how you view life, so set some goals, focus a little more on yourself, and take those steps show your parents just how hard you are really working.