Venting

Try to set some boundaries, and when they start to tell you about their issues, tell them you can't get distracted with other issues, sorry, as you have a lot on your plate at the moment. 

Positive mindefulness for me is to avoid people with stories that involve past unpleasantness

@Cora, it is so important to look after you. Getting breast cancer and the treatment is what your focus needs to be on. Maybe your friends rather talk about their issues because breast cancer is very confronting for some. I agree boundaries are critical and this is how to look after yourself. Be kind to them but do let the know your not up to hearing all this atm and when you are stronger you can get more involved then.

@Cora, You must be really nice person and have been listening to others. Time to have a break? It can be difficult, but facing breast cancer is tough. We need to look after ourselves even though sometimes I feel selfish. This stage of our life is not normal. I think if they are friends, they will understand you later. time will help? 
I looked after this lady she had a spinal injury and was unable to move independently and suffering from pain. During her hospital admission, her neighbour called her and stressed about the dog running away; when she hung up on the phone, we laughed. People don't understand what you have been through but some will. Maybe your neighbour, colleagues, an old friend of yours, someone will listen to you. 
All the best.

It is amazing how odd people can be.
I did get a little annoyed. For some it is almost like a competition of who had cancer, who had it more, what treatments and who has died. 'People' just don't think. It can be hard.

@Cora Seeing a therapist is a very good idea. I'm in the same situation when my sisters keep telling me their issues or asking me to remember something from last year, I ended up telling them I just don't have the capacity to solve their problems or remember anything right now. But I don't really blame them, it is difficult for people to understand how tough it is when dealing with cancer treatments. Only when you have experienced it can truly understand, so, seeing a therapist is the way to go to deal with all the stress. Take good care of yourself xx  <3