I am 47 and had a huge sex drive,now it really low,I hate it. I have a beautiful hot girlfriend and mentally I know from past she should drive me mad with disire.There is no problem with erection its just finishing the job.
Is it age or is there something else in the way?

Could be age,diet,mentally a block of one kind or another.. lot's of things make us go(or not as the case may be)

Lower testosterone can be tested for.

High Cholesterol can be tested for.

Diabetes can be tested for,etc..

If it REALLY bothers you, a lot... perhaps after having all these things tested,then go to the other doctors your GP might suggest.

In my opinion,If you can get hard,it probably isn't the medical issues, it might be more a mental issue,a block of some kind,a hang-up, stigma,etc.But that's just my non-professional opinion,just an observer.