I'm also in my 30s and get this problem, but only when I'm punting!
If I'm fucking my wife, no probs, rock hard and can go for ages.
Never had a problem with short or long term GFS (other than one but I blamed the 12 pints of Stella for that)
With a prossie, I struggle to coax it into life and then tend to finish before I've even really started. It's bizzare. Today I had a session with literally the hottest girl I've even touched, I had a semi at best till she started blowing me. Managed to enjoy a decent few minutes of hard doggy, then she wanted to change positions and it all went downhill. Ended up wanking my flaccid dick into a tissue whilst she pretended to make herself cum with a vibrator. This was the icing on the cake for a pretty rubbish punt, for all sorts of reasons.
Thing is, if I'd pulled her in a bar or whatever it would have been a dream come true