I am also a working mum and have bf my son for nearly 14 months.
I also went through this phase whereby I was really tired and wanted to give up. However, I realise that as my baby gets older, I can just extend the time interval that I express my milk. In fact, I can do it with 8 hours intervals, expressing once at work and once when I reach home.
I als dont't wake up in the middle of the night as it really tires me out.
Do hang on if you can.