I've noticed for the past 3 ish years now that when I try and masterbate, it takes me ages to get it up and when it does get up, it doesn't last very long.

I do have a girlfriend, but because of the distance, I only see her about twice a month.

Am I just bored of masturbating, after having something better (real sex) for so long. I don't think its an erectile dysfunction, because it stays up when I'm with her.

Its just quite a shock, when I think back to my teens and early- mid twenties when I couldn't be naked for more than 5 minutes with out getting it up.