now i am facing problem that if i hold left turn button or right turn button for too long, motor M will heat up very fast and it can't return to 0. is there any way to fix it ? 
can anybody help me

Which exactly M motor?

What is used for control?

If it is standard M motor, and if it is stuck in one positon, of course, it will overheat. U can try to use some clutch gear. And M motor doesn't have auto center.

Just use a servo motor

You could remove the thermistor inside, but it'll just extend the working time slightly. Also 2 rubberbands is an overkill, 1 is more than enough. Nonetheless you're better off using a 4€ servo from Ali, it is much better than the rubberband and hockey spring methods. If that's not an option, use a downgearing instead of return to centre steering, like this (kudos to legosamigos on bricksafe):

 

52 minutes ago, syclone said:

You could remove the thermistor inside, but it'll just extend the working time slightly. Also 2 rubberbands is an overkill, 1 is more than enough. Nonetheless you're better off using a 4€ servo from Ali, it is much better than the rubberband and hockey spring methods. If that's not an option, use a downgearing instead of return to centre steering, like this (kudos to legosamigos on bricksafe):

 

Well, I think it now not so far from the setting driven via the linear actuator...