First, you are allowed to decide if you attend a party or not. If you don't want to you don't have to. We understand your feeling you don't want to celebrate a sad event. Since you are the reason for the event make sure you give plenty of notice if you really don't want to be there.
However you might consider that your colleagues don't want to celebrate something upsetting (if they understand the full story). Instead they probably want to say goodbye to you properly, and to celebrate the many years you have had together. It would likely be good for you to say goodbye to them too.
Also you are about to enter a time of your life when you might feel isolated from other people. It might be good for you to take any opportunities you have for getting together with others - even if you don't feel like it. I understand you feel you can't leave your husband for even a short while, but I hope you can find someone who is prepared to look after him for a few hours. You will likely need someone in the future. As thursdaysgeek says, your plan to never leave the house for a period that may be many years is not sustainable.