@TaraV
Goodness you have a lot going on emotionally as well as the BC itself.
Easier said than done I know but try and set yourself one small task at a time.
As you say you are doing squats and lunges in the house. Perhaps make a note to yourself to do say 10 of each per day and build it up over a 4 week period to a number that you are comfortable with. Once you have achieved that then look at whether the walk with the pram can work for you and your little one (sorry don't know if boy or girl). Or another exercise like say a Bridge exercise, will help with the pelvic floor or if there's steps around a step exercise.
As you start to feel a little better then you may find the emotional crap isn't sitting so heavy! Sometimes they are throw away lines and other times it is an opinion based on perception and not knowing your reality so you just need to take a deep breath and say hey this is me I have some medical challenges but I am doing my best emotionally and physically to deal with same.
Personally I had excellent and still do, support from my hubby; he is the sounding board and whenever I would say I ran into so and so and the conversation was more of a competition about who had more treatment or whatever to do with BC he would just say I hope you just smiled and walked away!
It is very individual and your body is giving you challenges with Letrozole at present however if it becomes too unbearable there are other options that the Oncologist may consider.
Come on here as often as you can as we all understand and hopefully you will get better with it all and hopefully it is just a challenge that you can conquer. It's about coping mechanisms and strategies.
Take care and best wishes