I have had a few less than stellar moments in my sex life. Some examples that made a difference.
#1 I was on a diet and lost quite a bit of weight, helped alot. Enogh to wonder if I was mildly diabetic. I was on atkins and saw a change in my erections in a matter of days (darn near broke my wrist, whew!)
#2 Along with weight loss my blood pressure dropped a bit. I don’t know the biomechanics of it but I know that high BP can mess with erections. Arousal often increases blood pressure even more. Result: Little Count Drach dosent get to come out and play. (or was that play out and come)
#3 Feeling wanted, I have run into a few ladies who semmed to be taking their clothes off because, well they figured that was what they were supposed to do on the third date. Knowing you are desired, not just convenient always made a big difference to me.
As to the OP understanding and reassurance. Any guy who has had this happen is not going to advertise and more than likely is terrified that it might happen again, especially with a new lady friend. Combining performance anxiety with even mild medical problems that may impact erectile function can make for a mighty disappointing evening. Take it a little slower and you will recover. I know what works for me and if I feel my erection is “losing steam” I will try to adjust to give the proper sensation to keep it up. You may need to experiment a bit but with an understanding woman who you desire I’m sure you will win out in the end.