All that stuff you stress about, it really doesn't matter. The girl you like, go for it.. it's not going to work out in the long run anyway, it's not serious so just relax and have fun. Don't think of it like all or nothing, If she likes you great, if she doesn't then you lose nothing and it's not worth being upset about. Honestly in another 5 or 10 years you wont even remember the stuff that seems so life & death now. Now is the time of your life where you can make mistakes and still bounce right back, you don't get chances like that later in life so go for it.

Grades and college. Don't go into college with the expectation of it being like one of those movies about college, forget about the college 'experience' forget about this idea in your head about how everything will be awesome and you will be a completely different person. Go to college for 1 reason.. to get a piece of paper that says you finished your degree. Focus all your effort on the absolute minimum required to get that bit of paper then get the fuck out of there. The difference between getting 51% and getting 99% doesn't mean anything unless you are doing postgrad, so don't stress out and burn out on college.

The same is true for school grades, you use them to get into college, nothing else. Work hard enough to get into the college you want to go to, but there is no need to score higher than that.