If death is oblivion, non-existence, then why be afraid of it?
Oblivion isn't unpleasant. Nor is it pleasant. Nor is it - well - anything.
Perhaps it might be rational to dread the loss of something which you see as good, namely life, or your identity, or your 'youness'. But then again, once you have died, why would you care? Before you die, you still have life, and after you die, there's no 'you' to worry about it.
Surely you can't dread something that you're never going to experience? You can dread a dental appointment or torture because these are things that will be unpleasant for you to experience. But not existing will not be unpleasant for you to experience.
This might not convince you. Perhaps you still fear non-existence after death because you feel that not existing is somehow a bad experience. But Peter Ustinov had a point, didn't he? We've all 'experienced' oblivion before - because there's always been a time, a very long time indeed, when we didn't exist - namely, before our birth. Was that so bad? Do you look back on your immensely long period of non-existence with terror? No? In fact, the thought that we once didn't exist generally conjures up no emotional reaction in us at all.
Afraid of death? Nah. It's irrational. Don't fear an experience that (a) you will never have to actually experience, and (b) you have already 'been through', as it were, without it causing you any problems whatsoever.
And if you agree that life would be better if you weren't afraid of your future death, then it's rational for you to try to convince yourself that the above argument is correct; then your fear of death will be dispelled and your life will be improved.