You need to build up the muscles you need to push down the strings. Keep a clothes-peg in your pocket, and in idle moments do this exercise. Do everything as slowly as you can manage to educate brain and muscles. If it hurts, stop and try again the next day.
Step 1: First Finger
Take the open ends of the clothes-peg between the TIPS of left hand finger and thumb. Without any help from the other fingers, slowly close the clothes-peg together. Now allow it to open again - as slowly as you can. Repeat 5-10 times.
Step 2: Second, Third and Fourth Fingers
Repeat this process with each of the left hand fingers. Make sure the other fingers dont help. Make sure the fingers stand on their tips. The little finger will struggle - keep at it!
Step 3: Exercise the Gaps
Now put the open end of the clothes-peg between the base of the second and third fingers of the left hand. Bring the two fingers together to close the peg; open them again. Repeat with each finger in turn.
Then put the open end of the clothes-peg between the base of the thumb and the knuckle below the first finger of the left hand. Bring the thumb together to close the peg; open it again. Repeat 5-10 times.