It's not a problem unless you can't get erect when you need to, like when you are engaged in sexual acts.

Not having erections at unnecessary times is just part of growing up. You sound perfectly normal.

I am 25 years old and I just recently got out of the marines after 8 years. It was when i got out when I started to lose my sexual stamina. I dont have a problem getting an erection I have a problem maintaining an erection. I was wondering if it is added stress from getting out and the fact that I am having a problem finding employment. I do know that added stress can lower your sexual performance. I am very attracted to my girlfriend and she turns me on. I just dont know what to do to correct this. Thank you for taking the time to read this and respond. If anyone has any suggestions or ideas or knows what is going on please let me know.

Stress can definitely be a huge factor, and given your situation, it's probably the cause of your problems. You can try stress-relieving activities on your own, like meditating, or doing more of whatever makes you feel calm. If you find you can't help yourself, a therpist or even a sex therapist would be able to help you. I would suggest seeing your doctor to rule out any physical problems, and letting him refer you to someone who can help.