Hi darkside,
Welcome to the forum.
Fatigue is a very common side effect of the chemo. As someone very active myself and only in my fifties, it was likewise very frustrating to be too fatigued during chemo to keep up much activity at all.
As someone who has been there (I am now in remission), all that I can advise is to do what you can – especially as exercise is good for people with myeloma in many different ways – but don’t beat yourself up if that isn’t much. Your body is having to deal with both myeloma and chemo, so try not to be too frustrated and give it time.
Once treatment finished and I was in remission, I gradually became more active. I cycle away flat out in the gym, lift weights* and walk many miles. It took a while but fitness can be regained.
*I am relatively lucky in not having bone lesions. I checked with a physiotherapist and doctor first before restarting with weights as bones in myeloma patients can fracture easily (so this is an area to be very cautious).
Regards
Rabbit