Hi Amy, so sorry to hear about what happened to your dad. It is very much a shock when your dad can go from a seemingly healthy person to dying within a month, and it is no surprise that you are unfortunately struggling so much. You mention Diazepam, are you on this daily? Had you taken this before your dad’s sad passing? I ask because I have been taking it intermittently for years, but because it is highly addictive, doctors nowadays are very wary of prescribing it - in fact, I have had great difficulty getting even a few tablets every month from the local GP practice. So whilst it can be helpful for the short term, it is very important not to get addicted to it, and maybe this is something you can discuss with your GP.
As for the right way to cope, I don’t think there is such a way. People cope in their own ways. What I think you could benefit from is speaking to a trained counsellor - Cruse and Sue Ryder are two organisations that provide a service for those who have lost their loved ones. Counslling isn’t for everyone, but many people find they benefit from it. The other thing that can help is to keep talking about how you’re feeling, and so you have come to the correct place, because here you can post as much as you like and people will always be willing to help you.