Your FU was bringing your feelings into it after she told you she just wanted to be fuck buddies, and especially moreso that you kept messaging her and whining about the situation when she clearly wasn't being receptive. That tends to turn girls off, especially a girl like this who said she wasn't looking for a relationship. It makes you seem needy and insecure.

More to the point, it introduces drama that she was definitely not looking for. I guarantee you when you started whining about always being other girls' second choice, this girl was thinking "Uh oh, what have I gotten myself into?"

After she said she just wanted to be fuck buddies, you should have just said "I'm down." or something to that effect and left it at that. You said so yourself, you were having a great time with this girl, and you had the potential to get laid again. Why mess with that?

You need to learn when it's appropriate to express your true feelings and when to hold your tongue, not just when dealing with girls, but just in life in general. Hint: In most situations, less is almost always more.

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>haven't been able to maintain a relationship longer than a few weeks.

My guy, this is the reason why. Girls don't like overly-emotional, clingy, insecure, needy guys. You are a walking, talking turn-off machine. You need to learn to tone it down a bit and, for lack of better word, learn not to care so much.