It is a sad part of life, but we all will die at some point. Unfortunately, we do have to go through the loss when a pet or a family member dies.
I think that letting the fact that your pets will die stop you from getting them isn't really good. You could have a rabbit for a week or for 15 years. Even if it is just a week, you can still get a lot of joy and get very attached to them. There is so much to be gained from having animals around that far outweigh knowing they won't be around forever.
You do need to make the right choice for yourself. You might not be ready right now, but you may find they the right rabbit at the right time will make you want to open up again.
I have lost 3 rabbits, and each time have gotten another rabbit after. The amount of time between varied, but it was more about finding the right rabbit than the time. When my first rabbit Sophie died, I adopted Sean only about 3 weeks later. I wasn't seriously looking, but I did look and fell in love with him. I only had Sean for 7 months before he died and I would not trade that for anything. I did know that I wanted another giant angora, so I did wait to get Lillian. When Penelope died, I was no ready for another rabbit right away, and neither was Korr (her partner). It was about 6 weeks before I found Amelia.
Don't focus on death and loss as it will only depress you and make it harder. Focus on Whisker and making him happy. You don't know how much longer you have, so make the most of it. You can look into the options for when he does die, like cremation, paw prints and other memorial stuff if you think it will help, but don't forget that he is here now and needs your love and attention.