Howdy, I'm 18 and have had a fairly similar high school experience to you, with the exception that once I noticed that the girl wasn't going for me I dropped her and found a new one (which was successful for a while).

This may sound odd but I'm guessing what it really comes down to is that you are too smart for your environment. You mentioned you've never gotten a C before and that you are in AP classes. Now, I'll be blunt and say that I am indeed projecting myself a lot on to you but it seems to me like you probably haven't ever had to really work for any grade before. As and Bs were plenty sufficient to you, even if you took the hard classes you would get those probably by only studying for tests.

What I did to get out of this loop of just basically waiting around was to do things outside of school that interested me. For me this ended up being Theater, Computer Science and miscellaneous skills. Instead of going home and just doing the same old thing I did everyday (homework, then TV/Video games then sleep) I ended up with a lot more variance in my life. Even if I would spend only half an hour working on my Rubik's Cube solve time that was a half hour where I was truly engaged in something.

I think the "I never really do anything with my time" is what is the root of the problem here. I'll be honest with you again, I am totally balls terrible at sitting down and just doing stuff as well. For example, half an hour ago I told myself I would sit down and play 1 game of league of legends before taking a shower and going to do some work. But here I am! The only way to combat this seeming procrastination is to look at and realize what is causing it. For example, I know for a fact that once I get on a computer I will be on it until my next pressing appointment (and I will often be late to that). So what I have to do is say internally "Okay. Once I hit this power switch I am not doing anything else. That is a fact. I am choosing to give up my plans for this time slot to instead go on Reddit and be non-productive." So what you need to do is look at this last weekend and instead of looking at what you didn't do, look at what you did do and try to learn how to avoid what it is that is making you not do other things. If you were on reddit all weekend then just don't turn your computer on at all.

This works very well for homework. Don't say "okay 10 minutes of reddit, then homework" you have to say "when this computer goes on, my night is over." That's why I started to get home from school, eat some food, then immediately sit down and do my homework. If I took a break to sit on the computer then homework would become my last priority.

Okay, so that's what I think is the root.

Girls are weird. As far as that one girl you dated for a bit you made the right move. I have found myself both in the situation you were in where you were dating a girl you didn't like and also I've been in the situation of being in a stable, loving relationship. You chose right because first and foremost your girlfriend should be one of your best friends. You can't have trust, communication, and all that other stuff that makes a good relationship if you don't like or respect the person to start with.

As far as the new girl, she's just flirty. Yup, it sucks that people are like this when they aren't interested. Look at how she interacts with other people. She will most likely be flirty with them too. At college I have encountered 2 women in particular that after they talk to any guy the mans immediate reaction is "she's totally in to me". Something about just how they talk to people makes them appear very interested. I wouldn't worry about it. You mostly just have to realize that she isn't going for you, it's just how she is in general.

If the above paragraph feels totally wrong and it does seem like she is treating you different from others then she is probably closer to what Indianapolis_Jones was saying and she is most likely to be avoided.

You're also simply going to think about women a lot. Pretty unavoidable. The only times those thoughts get less intrusive (at least for me) is when I have a girl I'm either chasing or with who is clearly interested in me. At least for me that calms my mind because then I have someone instead of just being searching. Again, it's just sort of a fact and you shouldn't blame yourself for it or worry about it, just accept it. Nothing to be sad over or freak out about.

Let me know if you wanna talk or anything. Seems to me like we have a lot in common but I'm a bit further down the road than you are. I'm available for PM too if that's more comfortable to you.