[Ennui.]

I'd say that it sounds like there was a little bit of obsession, at least in the first relationship where you told the person you loved them after being in the relationship for about a week. That's moving a little fast to me. Some other things to consider are whether or not you are able to give the person space and not be with them at every hour of the day.
But, to an extent it is normal to want to be around the person and want to do things to make them happy. It's okay if you miss the person when they are absent from school, and that you worry sometimes about distance. Of course, you do have to challenge some of the thoughts. For example, if someone you like is hanging out with other people, remind yourself that it's okay, and actually very important, for people to have multiple friends and it doesn't mean that the person likes you any less. The same goes for distance. You can ask yourself if their behavior has changed because they're busy right now, or because they're truly being distant. Communication is important, too, so you can talk to them and say "I feel as if things are distant right now because... Is anything going on?"
I think it's a matter of watching how you act and how you feel when you are with the person that you are like. Try not rushing things, and make sure that you are able to take space so that both of you are able to hang out with other people. It's okay to think about the person but they shouldn't be filling most of your thoughts all day, every day. Make sure you aren't acting controlling of their actions/what they do, and be validating of how they feel.