Dear @Molly001
You have identified your own worst enemy, the tendency to overthink. I know that feeling well, but one thing we all learn during treatment is to preserve our energy and expending it on things that may never happen is a bit of a waste. So one step at a time. You'll probably get a bit of a pattern over three weekly treatments, so you will get to know when you feel good or not so good. You may, as others have pointed out, feel OK. I worked throughout two lots of chemo, because I was remarkably asymptomatic. Once you know, you can plan much better.
The loss of a breast is what it is. We just lost a part of our body with which we had a close, if not always entirely happy, relationship. It takes a while to work out what's best to do long term and there is no rush. You need to heal and take your time. Chances are you will be happy, like many other women who have been through this, with your final decisions.
Very best wishes