Emptynightmare;
Is that a defense mechanism?
In this last incident, did you respond as "I've used her up and she's not good for anything now." because you feel very sad that your friend decided to pursue her own dream of engineering? Because if you look at the situation as "I've used her up" then you would be denying your care and love for that person hence making it easier to deal with the emotional pain of the break up/end of friendship.
I just find it difficult to believe that you started this friendship with the intention of using her but when things stopped working out, you decided to look at it from the perspective of "I've used her up and she's not good for anything now." This perspective is cold thus makes the pain easier to deal with.
Also, Is it because you have a huge fear of abandonment and thus you have not been close to any friend?
I might be wrong. This is merely an observation.