My girlfriend and I sometimes engage in sexual activity more than once a day. At times, I find that after 2 or even 1 activity in which I reach climax, I have trouble maintaining an erection the next time around. It bothers me because I can't perform and it bothers her because I can't convince her that it has nothing to do with anything she's doing or not doing. This has occurred as much as 20 hours after the last sexual act. Is there anything I can do to correct this?

It is normal to have more difficulty in attaining or maintaining an erection after an orgasm. Most men find it impossible to have an erection very soon after an orgasm.

The period of time it takes to regain your arousal - your "refractory period" - is the length of time your body requires between orgasms. This period is largely predetermined by genetics, and it is affected by your age - the older you get, the longer it gets.

There isn't much you can do to change your refractory period. Assure yourself and your girlfriend that your refractory period is a built in part of your biology that has nothing to do with how interested you are in sex with her. But there certainly are ways to be sexual without having an erection!

While you do need an erection in order to penetrate someone with your penis, there are lots of other things with which you can penetrate your partner - fingers, tongue, toys - and there are lots of things you can do sexually that don't involve penetration of any kind. In particular, bear in mind that penetration is not the primary way that most women are sexually stimulated. The clitoris (the female's primary sex organ) is not usually well stimulated by penile-vaginal penetration. You can give your partner more direct stimulation with manual or oral contact.

If you feel a demand to "perform" by getting an erection, that demand will only make it more difficult to get an erection. The more relaxed you and your girlfriend feel about whether or not you have an erection, the easier it will be.

So relax, be creative, and enjoy exploring each other's bodies.

Erectile Dysfunction (inability to sustain erection) is a very common disorder which most men face in there life at some stage for one reason or the other like high blood pressure, high cholesterol, diabetes, fatigue, stress, physical injuries, certain medication, smoking and alcohol consumption. The exact reason behind your problem can only be judged by a specialist and only then one should use the medicines prescribed. You can have the information on ED and ED related drugs (Viagra, Levitra and Cialis) on http://www.levitrabliss.com/ . 
The best thing you can do is to consult an urologist, get to the root of cause and have the medication prescribed and surely you can get over your disorder.