That sounds really rough. First of all - if anyone thinks you are taking the Michael then they don’t know you so they don’t matter. Usually that kind of thing is just in your head. 
with work is it possible to ask for it as a holiday rather than a compassionate day? So taken from your holiday allowance? If that isn’t possible then just call in sick for 1 day and use it as your mental health day. You don’t need to be off for an extended period, just take the day. It might help to speak to a trusted colleague or manager.  I hope you can find a way through this. Whatever happens you’ll feel sadness, regret and lots of other feelings. You don’t have any control over what happens unfortunately, but if those feelings become overwhelming or affect your day to day then it’s time to seek support from your GP. 
All the best to you and your lovely mum.

I'm so sorry to hear this about your Mother ,especially the fact that she is so young its tragic , the way I'm seeing it is the important one here is your mam ,as you say she is your best friend and you do everything together so why not just keep on doing what you've always done ,after all you only get one mam you could get many jobs so if it was me I'd be concentrating on doing as much with my Mother as I possibly could ,work wouldn't even enter my head but that's me ,after your Mother has gone you don't want to have any regrets that you could have done more with your Mother instead of worrying about work ,just enjoy the time you have work will probably work itself out just try not to spend your time worrying about it your Mother is the important one ,I hope you can have a happy medium while your Mother is able .

Thankyou so much for the reply  I honestly wouldn't worry so much about work if it wasn't for the stress and worry about money and not knowing how much longer this will go on for. I live on my own and pay the bills entirely on my own with literally about 1k in savings which wouldn't get me far. All this is causing me so much stress and anxiety because I rely solely on me. There's only me to pay those bills and I don't come from a family with money so no one to ask for help. It's draining but I think I've made the decision to go sick. I have quiet a bit of time off coming up in March thankfully so if I can just get through Feb on sick it might be the only way I don't have a mental breakdown at work. 

Thankyou so much for replying :) You're probably right about things being in my head, I'm so bad with over thinking. Unfortunately I can't take it as holiday as I've used it all up. I was perfectly happy to take it unpaid but they said it's not an option. There's a lot of back story to it and I'm genuinely shocked they wouldn't let me take it unpaid but going forward I've decided I'm going to try get in with my GP tomorrow and possibly go on sick. I'm realising I think I need more than 1 day as I'm in a very bad way mentally. March at work won't be so bad as I have a lot of time of that month so I suppose going sick just helps me to get to a month where work and life will have a better balance if I'm making sense.