Questions, and some much needed help

OK, so i bought halo the other night, and for the past few days I've been playing it, both on-line and off, and i finished the story the other night, and now my disk is having problems.
So today I'm taking it back to get it replaced, now heres my problem, it seems as of late and its becoming a trend with me, i buy a game that I'm really excited for, i play it and after about 2 weeks or so i quickly lose interest in it. Now its not one type of game but all my last few purchases in general, including (halo, Forza 2, madden, and dirt) now i took back forza and dirt and bought something else with them but thats not the point, whta i need hlep with is, finding a game that either has very long and/or in depth story, or some kind of online play that will keep me comignback for more, so my question is, what should i get? I'm very desperate because i feel hopeless as a gamer, i mean if i am tired of halo 3 already and Ive owned it for less than a week, should i feel like theres no hope left for me as a gamer, or maybe I'm not finding what I'mtruly looking for in a game. So please all the help or advice is welcome, but please no flaming, i didn't say halo was bad or any other game, but i truly want help and advice on my topic. thanks to everyone who helps and offers advice.