I am so sorry to hear your news and also that you are feeling that you could have done more.This is a completely natural way to feel but I am sure you did all you could and you were there for him which is what matters most to our men. Some cancers are just so aggressive and all circumstances are different with men reacting differently to treatments. If he could have been saved I am sure he would have been, but you shouldn't blame yourself.
I do understand, having lost my husband to an aggressive cancer seven years ago. Other men had been fighting for years and my husband lost his fight 20 months after diagnosis. I felt how you describe, but eventually I managed to forgive myself and I hope you can too. It suddenly dawned on me that if all the experienced medical folks couldn't stop the progression of the disease how on earth should I think I could, but at the time I did feel I had let him down.
In time I drew comfort from knowing I was there for him and knowing he knew that. Today's painful memories and thoughts do turn to a more comfortable feeling, but it all takes time.
If you feel it would help to talk to someone I am happy for you to message me, or the Specialist Nurses on 0800 074 8383 can put you in touch with someone who has 'been there'.
Take care, Janet