I am a 69 year old dog with sag rising. Discovered that I have a brain tumor last year

Oh, don't worry. This year is much better than last year. In the daily life, you should have health diet. Besides, you should have regular time to take a rest. If you can, it's also suggested to take more exercises in order to help you recover soon.
You need to become happy. Everyday is a new day. You can make full use of time to do the things you really like. Don't have regret.