There are many reasons with regard to not maintaining an erection, even if you are turned on.
Being on certain medications, having diabetes or other illnesses, having anxiety problems, drinking too much alcohol, lack of sleep are just a few problems that can cause erectile dysfunction.
Make an appointment to see your doctor to run some blood tests and check your prostate.
35 years old...Any Medical problems? Psychiatric Medications? might just be time to try one of the prescriptions (Viagra, etc...) Although they seem to be causing other problems now. Quite honestly....are you doing what your sexuality dictates? Talk honestly to a doctor you're comfortable with...see what he says. You don't want to continually miss out on one of life's true pleasures. I wish you the best... Andy