Carly, you are like a my long-lost sister from another mother. I am exactly the same. I call myself "Jill of all trades, Master (or should it be mistress) of none!" I have so many interests and constantly have to replace one with another as the interest wanes for one reason or another. One one hand I want to get certified as a yoga teacher, I want to get certified as a life coach, I want to get back to dancing, I want to get back to Spanish lessons, I was to start a cooking experience for people. Sigh! It's endless!!!
And yes, I too don't quit on relationships or writing or tennis.
You are not alone. Thanks for sharing your dilemma. I don't think we are quitters. We are just passionate people with varied interests.