Ever have this happen to you?

1) yes and no: yes (I get bored of or frustrated with one game at a time), and no (I like to focus on one game at a time to completion... but the "yes" gets in the way)

2) TF2 is probably the closest in recent history.
A couple from the past that I played a lot would be TFC, Counter-Strike (1.6 and lower), Sim City 2000, Total Annihilation, Starcraft with friends, and Jagged Alliance 2

3) I always try to avoid pay-to-win, and haven't yet done any pay-to-plays, but in the free-to-plays Champions Online was fairly good, but there've been a lot of changes and Tribes: Ascend is pretty nice and I need to get back into that one

1. yes, but only after i feel that i completed it

2. TF2

3. You know

1) No. But it's hard not to.
2) Eve Online. I've been playing that since '06, but it may not suit your tastes. Also Minesweeper.
3) Pay to play (or F2P, if you can get enough ISK). Also F2P.

As a sidenote; There are plenty of games that I'll return to after burning out on 'em. In all honesty, I think all games are like that for me. The TTB (time 'till burnout) is different, though.

I think co-op games are the easiest one to play forever. My favorite one is Payday the heist. Had to stop playing it beacuse I'm offline most of the week but get 3 people that know what they are doing and you'll be playing the same heist for days.