My girlfriend and I have been having sex about once daily, and lately my penis won't stay hard during sex.

For the first six months I had no problems now all in a sudden I lose my erection.

I do about 200 push ups every other day and 200 sit ups every other day.

I do not have high blood pressure or diabetes what can it be ?

Maybe try not having sex for a few days. Also maybe you could try a cock ring.

Try to spice up things in the bedroom, or having sex less often.

Perhaps it is just too much, lack of spark. It can be stress, esp if you once have a issue, all of a sudden you start worry next time and the next

Talk to your doctor, there are great pills now, Viagra, cialis etc that work like magic.