I have a good amount of experience with obsession. I used to become obsessed with people quite regularly ever since i was about 14. I could almost define my life looking back, in terms of who i was obsessed with.
As Fireworkz was pointing out to you, it's all about some underlying fear or conflict going on with you that the object of your obsession reassures and soothes with their attention (or the idea of possessing their attention).
What you need to realise is:and I can't help it cause I love this guy and he loves me
You can't really measure your love by how addicted you are to something. In this case, you bf's attention.
You are experiencing this person and the highs and lows they create in your life like a drug, which isn't bad unless like you say, it is causing you harm.
Figuring out what you enjoy so much about this relationship and why you fear losing it so much, will help you understand how to control your emotions.
If you chalk it all up to "i love him" "he loves me" you will not learn anything, because you will be making him "irreplaceable" and your "love" irreplaceable which is a nice thought, but absurd and childish.
I see that you are 19, relationships are almost supposed to feel this way at 19. I envy you.
And i suppose i also pity you as well.
Just find a way to believe that no matter what happens with this relationship, you can handle it and everything will be ok. If he cheats on you, you will get over it. If he dies, you will get over it. If you live happily ever after, you'll be able to deal with that to. Just find a way to trust, that no matter what happens, you will be alright. If you can trust yourself to handle anything that can possibly happen, you will not have to spend so much time worrying about what will happen.