Oh, I'm so sorry that this has happened to you
Drug errors are bound to occur on occasions and we've all made them in our time. It only takes a moments distraction or loss of concentration and that's when it happens.
Try not to beat yourself up about it. You're bound to feel upset, guilty and stressed - it's only natural, but the error doesn't mean that you're a bad nurse.
The RCN are very supportive and will help you through. Your colleagues will fully understand the pressure that you're under.
I doubt very much that the guy died from the Amlodipine administration - he'll have obviously had other problems or he wouldn't have been in hospital in the first place.
I'm sure that you'll be fine and that you'll learn from what's happened and move on. Remember to chase anybody away from your trolley if they disturb you in the future!