Research suggests that daily tadalafil (Cialis) can aid nerve repair by drawing oxygenated blood to the area - starting dose is often 5mg but 2.5mg tablets are available if you find the side effects problematic. There is no such research to support the idea that sildenafil (Viagra) aids nerve repair. Unfortunately, quite a lot of CCGs have blocked the prescribing of tadalafil for nerve repair which means you get offered sildenafil or nothing.
Tadalafil also comes in an 'event' dose as well as the low daily dose.
Under NHS guidance, men who have ED as a result of prostatectomy are entitled to a vacuum pump on the NHS and / or as many event doses of Cialis or Viagra as you need but a) many CCGs have blocked the prescribing of pumps and b) limited the prescribing of tablets to 4 x per month ... in effect, they have decided that you can try to have sex once per week.
If you get a few months down the line and have not had an erection using tablets, you may then be assessed for injections - again, it depends on your CCG. Where we live, you have to fail with all types of tablets, cream and pellets before you can try the injections.
It does seem that you have been rather over-optimistic on this but I think that is understandable - so many surgeons gloss over the possibility of ED. My husband was 50 at diagnosis and our surgeon spelt out the risk & likelihood very clearly but J simply didn't take it in because he was thinking 'well I am only 50 so I won't have any problems'. It took 3 years to really make any progress and 11 years on, he is still dependent on injections and tablets for penetrative sex; not because of any mechanical problem now, I suspect, but just because years of problems have completely destroyed any trust in his own body not to let him down.
Your starting point is probably to ask your GP or surgeon for a referral to the local ED / andrology clinic (if there is one in your area).