Whoa whoa whoa! slow down! I hope you haven't gone and done the deed just yet because you have an awful lot of questions, and for that I'd suggest you wait just a little bit longer till you find out all the things you need to know. After all, sex is a pretty big and important undertaking and it helps to have as much information as possible.
First off, here are a few links that will answer your general concerns:
I know that's a lot to read, but you asked a lot of questions. And I suggest you read it with your boyfriend, too because I'm sure you two have a lot to talk about.
Oh, I forgot to look at where you were from, but please check out http://www.ageofconsent.com to see if it is even *LEGAL* to have sex with your boyfriend where you are. If you are too young by law to consent, your boyfriend could land in a *LOT* of trouble (which may include jailtime in severe cases).
Other questions you have:
Will you die sooner if you are a virgin? No. But you will die someday. Everyone does. As for your fear of death, I recommend you talk to a clergyman or read some books about death and ideas about the afterlife.
Will you think about sex more often? Not necessarily. But your thoughts on sex may change a little after you learn more about sex from your experiences.
Will your grades slip? Part of being responsible about sex is being responsible about the other aspects of your life. Your grades will not slip as long as you study hard and keep your time evenly divided between your schoolwork and your lovelife. In short, as long as you don't spend all your time screwing around, you should be fine. And if your grades do slip, it's probably a good idea to *STOP* having sex and get back on track with school. That's right, once you start, it is perfectly OK to stop.
As for other things you said, sex is NOT and essential component of a relationship. Having sex does not make you mature. It does not make you a better girlfriend. And it certainly does not guarantee unconditional love.
If there's any doubt in your mind, please listen to it. You will have an entire lifetime of opportunities to have sex. You should never feel like you have to rush sex. Haste makes waste. If you hurry through sex, you can forget inportant things like safety, legality, or even your own real emotions.
Correlation does not equal causation.