Blast!! Feeling low seems a perfectly reasonable reaction - as you say, you thought this was all behind you. If you have a non-sentient object you can use as a punching/kicking bag, let rip
on it for as long as you have energy. Then deep breath and do what you have to do, again, one step at a time. As you say, the cancer lens can be a gloomy one so look at the world in the light of what you have, love and value. Those feelings are important, not just to you. You are better informed and prepared, you can do it again. Very best wishes.