I need a win in my life

Dungeon Defenders

easy - medium? you can always play with yourself;)

you will feel accomplishment at the end:)

Achievement Unlocked because it's so satisfying getting congratulated even if all you did was press a button and it's free! Think there's a sequel too

play that
you will feel like a winner at the end...

http://www.totaljerkface.com/happy_wheels.php enjoy ;)

RPG? The Witcher 1 is easy on normal difficulty.

dungeon of dredmore

research and development mod for hl2, most rewarding thing i ever played

Qube, pretty hard puzzle game i think but worth the play.

Just go easy setting on a game..

i just play online games to get my satisfaction.

Go for casual flash games, you can find a bunch of them here. They're pretty gratifying and usually provide a medium challenge.

Additionally, there are priced casual games such as World of goo, Borderlands, TESV:Skyrim, Left 4 Dead 2 and Call of Duty that can provide accomplishment as you achieve over nothing.