Is it normal not to be able to stay friends for more than 2 years?

I had alot of really good friendships in my life so far (I'm 22, male), but they all broke apart.
I believe it was always my fault...

My first best friend was a classmate from 3rd grade to 6th grade. I flunked 6th grade. First we still hang out between lessons but then I fell in love with a girl from my new class and didn't take time for him anymore.

My second best friend was that girl from the new class that I fell in love with. She was the best and the worst thing that ever happend to me. She was in a relationship since long before I got to know her and was obviously just looking for a good friend, wich I was for two years until I couldn't pretend anymore.
I flunked again in 8th grade, this time on purpose to get away from her.
I still miss the friendship we had and I still love her too, although we didn't talk or see eachother in more than 3 years now.

My third best friend is a girl I met over the internet. She still the best among my friends but only because of lack of competition.
We used to talk about everything everyday and she helped me alot dealing with my broken heart, but now our conversations keep getting rarer and shallower.

Another good friend of mine is another girl ex-classmate. I'd really like her to be my next best friend but unfortunately I think parts of me want more.
She has lots of similiarities to the girl I fell in love with: shes sweet, funny, kind AND she is in a relationship since many years. I didn't tell her that I like her more than just as friends because I don't want to trouble her, but I'm trying to take a timeout from the friendship so I'm avoiding her.

So once again, I'm all alone, without a friend and still single.