Okay so I have a sixaxis model of the ps3 controller and I like to play a lot of FPS (First Person Shooters) as you will know you have to press the L3 button to sprint in games such as MW2,MW3,Black Ops etc well the problem is everytime I press L3 I sprint and stop straight away unless I press down really hard and so it's uncomfortable and annoying doing this also I think it's had some kind of impact on the analog stick because I'm know finding it harder to walk forward without it stopping on games when I don't even need the L3 button. What do I do? Any Suggestions?

Unfortunately you have essentially deformed the internal components causing it to have erratic readings. You have a few solutions, you could call Sony and see if they have any warranty on the controller. You can use a different controller. You can buy a new one. Or you could be cheap and scam a store by buying a new controller, then clean your broken controller up and place it in the box the new one came out of and seal it back up, then return it saying it had a problem and say that you just don't want it anymore.

If you do get another controller, be gentle, you dont need to push that hard for the button to work.