I am really sorry to hear about Heidi, at 11 she has had a good long life, hopefully you can enjoy her for awhile longer and spoil her rotten.
We lost Ray just before Christmas, he had heart problems. We did have him on heart meds for a few months and that really made his last days alot better, he had to go through alot of testing to get the right meds, at the time we didn't know he wouldn't last very long, but he was happy to the end. I just wish we could have had a few more days with him, he just went down in an afternoon and when we took him to the vet they said he wont make the night without a transfusion, then tests to see what was wrong, and then he might only live a few days longer. We didn't want him to go through that in his last days so we let him go.
I wish you all strength and lots of good boxer loving days, make the most of the time that you have left with her, don't worry about tomorrow, you will do what you have to when the time comes but try to enjoy the now.