First of all...the hardness & erect state of the penis is determined by the blood flow in that organ.
And the medications like Sildenafil
citrate (Viagra) maintains erection by increasing blood flow to the penis.
However...performance can be improved by having a diet rich in fruits, vegetables, fish, nuts & honey.
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 or improve performance...it may be harmful.
Regarding the problem of difficulty in insertion, it may be due to anxiety
or stress due to which there is reflex contraction of the muscles of vagina & pelvis..
So...tell your partner to stay relaxed during intercourse.
Best thing is to eat a healthy diet, exercise regularly, avoid stress, get good sleep, think positive and stay happy.
Wishing you Good Health.
Thanks and take care.