HUGS
It is such a whammy when you come up against a life-shortening issue that is beyond your control. I have had two piggies dying from a bladder stone related problem - and not for lack of trying, so I really feel for you.
Concentrate on filling every day with love and enrichment to make it count. Guinea pigs don't have a concept for life expectancy; they measure their life in happy todays. You can pack a life time's worth of love into just a few minutes.
I have adopted piggies who were with me only for a short time but watching them having the time of their life means that I will always remember them with a smile and that the time they have been here seems to be much longer than it actually was.
What you are currently going through is the onset of the grieving process, which doesn't start with death but with the moment you fully realise that death is on its way. It is the second most lowest point you will experience apart from the actual goodbye. Give yourself time to grieve and allow yourself the freedom of living the remaining time consciously and fully - this means that you have actually done a lot of the emotional processing homework beforehand so you will be still very sad but you won't experience all the other baggage to the extent you would normally do and you will have a somewhat easier ride.
Grieving dynamics are different from those of a sudden death; overall you are not grieving any less. But you have far more control over how bitter or how sweet the remaining time is and how many precious memories you can create to fill that special Buzz-shaped nest in your heart with. From my own experience of losing over 40 piggies in all sorts of circumstances plus my dad after a prolonged battle with terminal cancer I can assure you that it really makes all the difference if you approach the coming time with an open mind and concentrate on the quality instead of bemoaning the quantity. Buzz may not have the longest of lives but he can still have the best one you can give him.
I know that this guide doesn't make easy reading but the chapter on looking after a piggy that won't recover will hopefully help you get through this difficult time as best as possible and with the feeling that you haven't left anything lying on the floor:
A Practical and Sensitive Guide to Dying, Terminal Illness and Euthanasia in Guinea Pigs