You are absolutely right that becoming obsessed with girlfriends and becoming possessive and jealous are problems.Relationships are supposed to be sources of joy, not sources of anxiety.You mention that you are anxious and intense and it is very likely that these two traits lead you to feel insecure about your relationships.
I am very impressed that you are so self-observant and concerned that you may be doing something that is harmful to both yourself and the females in your life.
Just so that the readers out there know what was written about in the earlier article to which you refer, I'd like to fill everyone in.
In that letter, a mother was concerned that her teenage daughter's boyfriend was obsessed with her daughter, extremely jealous and overly-possessive.
In my response, I shared and validated the mother's concerns.
You see, you may inadvertently be causing the very thing that you are afraid of.
My guess is that the girls get frightened when you get excessively needy and clingy and it sounds like you are not happy about this behavior, either.You ask what causes such obsessive behavior in relationships.Before I elaborate on the cause of my concerns, I should mention that I write this not as a concerned parent, but rather as a male in his late teens describable as anxious and intense.The details of my past and present relationships with females struck me as being scarily similar to those described in that inquiry.My actions in that regard always struck me as being very, very wrong, hence my reading that post to begin with.I describe it as being almost addiction-like, with very real withdrawal symptoms.