Hi there lucy ... so sorry to hear your dad's prognosis ... my hubby daughter, went through similar to you with his late wife ... she was at college and only 17 ... she found it hard to stay in the house, so went out quite a lot, as that was the only way she could cope, as it was all overwhelming for her to see her mum so poorly ... and having breaks from it helped her through ... 

How is mum coping ... can you both tell each other what your fears are ... maybe you could work out some sort of Rota, to give you both some "time out" and mcmillan maybe able to say if theres any groops for young ones to chat ...  and they have a free phone number ...

All l would say, is try not to think he has ? Long left... say instead, he's here today, well make the most of today ... that way you will make every day count ... that's what I'm doing on my cancer journey... you have time for him to tell you his story from being really young (we all have a story in us) and say what's in his heart .. 

I look at every day as a blessing ... and try and find something good in every day ... even though I've had a few "loose it days too "  but if you can hold his hand and walk this path with him, you will have some good memories as well as sad ones ... feeling as you do, is really normal , so be kind to your heart... wer here if you need support along the way ... Chrissie

Hello lucydoc.  As well as the helpful information which Chrissie has given you if you type the words "anticipatory grief" in the box marked "Search Forum" you will find quite a few people have also posted to talk about this.  I am sorry this is something you have to discover but thought you might like to read how others have coped.  Best wishes.