Norman,
There are things you can try, and it's worth asking for an ED clinic appointment unless your GP is knowledgeable in this area. As a PCa patient, you are entitled to referral for ED.
It certainly is worth asking to try out a PDE5 inhibitor, such as sildenafil (Viagra). You do need some arousal for it to work, but it amplifies the effect.
You can look into using erotica or soft porn which can help some people, but you must be careful it doesn't damage your relationship - it would need to be something you both agree to, and not leave one or both of you uncomfortable.
You might look at doing some things you haven't tried before. Addition of sex toys such as vibrators might help. Also, if you have traditionally lead, try swapping roles, possibly combined with S&M restraints, handjob/blowjob, etc
Expect some of the things you tried to fail. Be ready with kisses, cuddles, touch, etc. Think back to things you might have done in teenage courtship before full blown sex, and play those out again. Some people find that can be very rewarding, and take the stress out of performing sex.
Sex may change, but don't assume it's over yet.