I don't know if this will help, but it won't be behind you by year-end regardless. You have the initial surgery done, but there will be weeks of follow visits, drains, etc. As far as dealing with change in general, I think that is mostly a matter of perspective. It is pretty much the norm for things to change whether it is cancer treatment or anything else. Stuff happens that requires us to adapt and change plans every day whether it is our jobs, our family life, our health, and dozens of other variables. If we don't realize that and have backup plans, or are able to adapt to change, everything will always be a constant roller coaster that is created mostly by being too rigid and expecting things to stay the say or as we plan them.
I was diagnosed just over a year ago. I'm am just now at the point where I don't have some medical appointment or treatment more days than not. But even so I still have regular blood tests, appointments with my oncologist, monthly port flushes, and other medical appointments and treatments for various side effects and other conditions resulting or exacerbated by my cancer. I have accepted that this is now part of my life, my "normal" and because of that it doesn't stress me out any more.