An option to consider if you're finding the transition hard is to get a controller to use with your PC to help you ease into things.
As far as getting in the mood to play games goes that's a bit trickier. What sort of games do you usually like to play? Racing, fps, rpg? Do you listen to your own music while you game or is the game's soundtrack important to you?
A good idea for getting into pc-gaming might be to check out a bunch of indie games. They're often cheap or free, short enough that you can play them for a while and then move on, and they often have unique quirky play styles.
What are the system specs for your PC? That will help us suggest some games you could run that you may be interested in.
Wired xbox360 controllers are cheap and easy to find. As the others suggested, many of the indie games on Steam are cheap and a lot of fun. You could also try hooking your PC up to your TV if you prefer gaming from the couch...
I'm both a PC and Xbox fanboy, I know the struggle. A lot of people usually recommend Steam for the amount of games on it, but not every game is on Steam. Suprisingly, there are a lot of games on PC that are also on console. Here's a list of games you could look at:
Battlefield 4
COD: Ghosts
XCOM: Enemy Unknown
DayZ standalone
Minecraft
Planetside 2 (F2P-FPSMMO)
Counter Strike: GO
Star Wars: The Old Republic (F2P-MMO)
Skyrim with mods haha
Crysis series
Assassin's Creed series, IV: Blackflag just came out recently as you obviously know
Bioshock infinite
Mass Effect trilogy
I realize I posted quite a few games but there are some cool ones out there if you know where to look. The gaming mood comes and goes, generally when you're really bored or you feel like just saying fuck society for a few hours, forget everything, and load up a game.