Sounds like you just ease off the gas when you`ve built up a good
lead.....common problem many players go through to be honest.
What happens is people work hard, they get a great start then
without being aware of it they lose the hard work ethic and think
the match is in the bag....
They switch off and before u know it you`ve lost the set.....
Its a bit like having a tug of war....you work real hard on the rope
and ready to win but you ease off the tension in the rope thinking
its to easy.....when you ease the tension your opponent takes it
up.......before you know whats happened their running away with it.
You have to stop yourself and think what did you do to get that
lead.....then use that!.....
........don`t change a winning game.....!
Hope that helps