My 300cd 82 starts but it does not pass 30mph

[diesel boy]

Hello I think I got a huge problem. My car starts and the transmision is very ok because I can fell it changing gears. I acelerate it pedal to the metal, all the way down. The car starts to move slowly until it reach 25 to 30 mph and a little more but is very hard to brake and to acelerate. What is this problem and is it hard to fix please help it might die on my hands. And That's is not a option I trust you guys. Thanks Also it does not turn off.