Ok, here's my take on it, just my opinion. First, this problem is more common as we age. (General statement #1) I think if we polled it, we would find a significant number who has difficulty gaining a climax as they get older. Some easy things to blame would be boredom (sorry, girls) , worry that we might not be able to perform, resulting in a self-fulfilling prophecy, loss of sensitivity after major treatments (remember, I'm speaking of possible experience of prostate cancer guys, who can have very large and serious ED issues with any of the various treatment options, probably much the same with some options for drug side effects from Parkingson's treatments. That's just general possibilities, and not all of them.
Now, it has been my experience with trimix that it does have a slight numbing effect when I use it, I think in my case, it's the alprostadil in it, which is known to cause aching and or pain, so I always get a slight amount of the aching which can be distracting when having sex. This, I suspect may be your problem. You haven't had a lot of successful episodes yet, so this may not amount to much evidence as you continue to use it. I would suggest that you continue decreasing the amount of trimix to the point that you can have a repeatable erection lasting around an hour. Maybe with less use, the problem will solve itself.
There is another alternative, if you wish to try it, and if you think the trimix is causing a problem big enough to go with something else. As you probably know, the bimix is the same thing without the alprostadil. If it worked to give you the erection, you might find that it doesn't affect you the same way. You use more but have less side effects.
I hope this has given you some ideas to followup. Good luck, report back with results...
James C. Age 63
Gonna Make Myself A Better Man tinyurl.com/28e8qcg
4/07: PSA 7.6, 7/07 Biopsy: 3 of 16 PCa, 5% involved, left lobe, GS6
9/07: Nerve Sparing
open RP, Path: pT2c, 110 gms., clear except:
Probable microscopic involvement-left apical margin -GS6
3 Years: PSA's .04 each test until 04/10-.06, 09/10-.09, 01/11-.09