I'm also surprised by this:
There is a very good reason why anyone who's had cancer is classed as disabled in terms of employment for the rest of their working lives.
OP, I was diagnosed with Stage 3c breast cancer in 2012, radical mastectomy, axilla removal, dose dense chemotherapy and 25 sessions of radiation, followed by aromatase inhibitors (for 5 years) I had 8 months off, then returned to work part-time about 8 weeks after I finished radiation. Like you, I struggled early on, I was so tired and so shell-shocked by what had happened to me. My colleagues were great, but honestly they had no clue how I was feeling. They threw a morning tea to welcome me back, and I took that opportunity to talk to them about how I felt. I spoke for about 5 mins, quite honestly and openly, about the reality of my physical condition and how difficult things were emotionally. For me, that worked wonderfully as everyone got a very honest and clear picture of the fact that I was not 'fine'.
But here's the thing - almost 7 years on from diagnosis, and things are very different. I have been promoted to a very senior position, I am also studying for my masters, and I am feeling fine. Apart from being minus a breast and having osteoporosis, I am physically doing really well. Mentally, cancer no longer owns me. I only think about it rarely and fleetingly. It doesn't interfere with my daily life at all, and is not a consideration in any of the choices I make - with the exception of the osteoporosis, which means I can't do any sports which would put me at risk of a broken bone. But honestly at 49 I probably wouldn't be doing that anyway LOL.
Things are very tough for you right now, and I don't want to downplay that. Yes, you should let your colleagues know what it's like for you, and do that in whatever way makes you feel comfortable - if that's via email, then fine. I am sure they will appreciate knowing and it will help them to understand your situation and support you in lots of small ways, which on a day-to-day basis is really important. But as a long-term survivor, which given the amazing stats with breast cancer you may well be, please don't think that what you're experiencing now is how you will always feel. Physical and mental recovery takes years and it's a process, but you won't necessarily experience long-term and/or significant disabilities because of cancer.
Best wishes to you.