I scored a 57ball 100 in a t20 and only hit 1 six!
You don't need power and sixes, it's just obviously nice to have them.
Try some range hitting. Get on the square(old pitch/artificial) And just try and clear the ropes.
You obviously have to get under it, but just keep.trying to add more bat speed, while maintaning the timing and shape in your shots.
Hopefully muscle memory will help the extra bat speed become regular.
If all fails, learn to judge and nurdle, move the ball around to fielder less spots. Don't stay static in your crease, move around (forward/back, over to off back to leg etc) and up your run rate that way.
We have a half decent t20 side. Unfortunately we have 6 massive hitters. And then a few guys that are made for test cricket. So if the 6 fail. We struggle.
Would kill to have a nurdler or 2 in my side.