Thank you for all your tips
you may be experiencing erectile dysfunction. as with what i have read if it occurred once or just few times there is nothing to be concerned of but if the problem continues, go see a doctor. the problem may be caused by stress from your work or other health problems. so try reducing stress. but i suggest if instances persists, go see a medical profesional.
Cayopteris has some good tips for you to try.
I would also suggest that you make an appointment to see your doctor to have your hormone levels checked. It is very common for men to have low testosterone levels. If this is the case (confirmed by a blood test) the doctor will be able to prescribe testosterone gels that are applied to your arm or leg that is absorbed through the skin.
The doctor will also be able to check your prostate to make sure everything is fine.
You could also trying giving yourself a massage in that area to help with your erections.
The other thing to consider is whether you are taking any medications or have diabetes. Some medications can cause erectile dysfunction and if you do have diabetes and have high sugar levels that are not well controlled that too can hinder your "pleasurable functions".
Do make an appointment with your doctor to discuss this.
The other problem may be a psychological one, but as you are "eager" to have an intimate relation with your girlfriend, I doubt that is the case, but well worth considering if all else fails.
All the best.
Make sure you are exercising. Running and pumping some weights keeps your drive and testosterone up. You could help yourself by taking L-arginine for blood flow. Pumpkin seed supplements can have a good effect.