First of all remove all myths & misconceptions from your mind about masturbation
In fact, Masturbation is completely normal & doesn't have any bad effect to the body and mind.
Regarding the problem of decreased erection of penis, it's mostly due to decreased blood supply to the penis.
It can be improved by having a diet rich in fruits, vegetables, fish, nuts & honey.
Avoid stress...practice meditation
..this helps in improving concentration & builds up the confidence level.
You can also do oil massage
of the penis...it increases blood flow & maintains the erection.
One more thing...avoid consuming any over the counter medication
which claim to increase the size of penis or improve performance...it may be harmful.
Best thing is to eat a healthy diet, exercise regularly, avoid stress, get good sleep, think positive and stay happy.
Wishing you a Healthy Life.