so I think we can deduce your libido is fine. Difficulty getting and maintaining an erection is down to one or two possibilities then ...
a) work related stress / other stress
b) ED as a result of radiotherapy
Option b) tends to emerge some years down the line; John went to see the GP just before Christmas to discuss as he is having the same problems you describe (his RT was 6 years ago). Amazingly, a small change of tablet has been amazing - he has stopped the Viagra and Levitra and gone to 20mg Cialis with great results. You might just need a shake up of your tablet combo. I think in your situation, taking 4 Cialis tablets together is worth trying before you ask for any change to prescription. Also maybe try Levitra, which we find works slightly better than Viagra.
If it is nothing to do with the RT and everything to do with option a), I don't think there is a tablet in this world that can sort that out for you so perhaps you need a bit of time to address any crap in your life that you can do something about.