Hi there exhausted
8 months in with a partner with GBM you have a right to be exhausted, sad and angry. I don't have a brain tumour. I know people who have and continue to keep going several years in. I also have had friends who have been carers for people with a GBM and they all tell the same experience - exhausted, sad and in some cases angry. You do need to get some support for yourself - it is out their but you do need to hut for it. When your exhausted that can be difficult. Start with the Cancer Helpline 13 11 20. In some states there is a Carers Cancer Connect to put you in touch with others in a similar situation. There are also organisations that do provide respite. I can remember one of my friends talking about how much of a relief that was when her partner agreed that she needed some respite and she was able to take it form the organisation offering. Ask to talk with the social worker at the treating hospital for your partner.
Just a few suggestions, made a bit in the abstract as I am not in your shoes.
Take Care
Sailor