>I start to fake a cheerfull, positive, easy-going personality

I says it right there. Right from the start you go into "fake" mode, playing a conquering game. Once your mission is completed, the fun of the game goes away. It actually makes a lot of sense when you think about it.

I'd suggest approaching people you think you might like in a less fake way. Try to see social contrct more as a mutual trade between two equals, instead of you selling yourself to the other. When the friendship is based on something you perceive as more "real", it might last longer without getting boring.

Or, alternatively, you could come to terms with the fact you just like to friend-hop, and enjoy your little social theater performences. It's your choice really, I can see how both might be enjoyable.