Hi
BIG HUGS
I am so very sorry that it wasn't to be!
Please try to take consolation that Buttercup is no longer suffering from some obvious discomfort that would have only got worse and killed him in the near future but with much more pain and unpleasantness, seeing how quickly it had come up. You haven't failed him in any way.
Buttercup has also lived a normal average life span - and quite obviously a happy life in your good care. Try to take comfort from that. When and what from our beloved ones die is not for us to choose and out of our control. Guinea pigs measure their lives in happy todays; they do not have a concept for longevity or average life spans. As long as you have given him lots of good days, you are well ahead as far as he is concerned.
Guinea pigs know very well (and often much sooner than us) when a guinea pig is ill and when a companion passes.
But - like humans - they can react very differently.
As long as a bereaved companion is eating and drinking as normal (which is thankfully the case in the vast majority of bereaved piggies), they will generally keep and come out of deep mourning after a few days, if without their usual sparkle. Some doughty souls will simply carry on as normal. In all these cases you'll have the time to look for a suitable companion; in your case a de-sexed sow (unless Truffle is already de-sexed himself). The ideal period is about 1-4 weeks but it is always worth to wait that bit longer and go further if it means the most long term happiness. Last year, I had to wait for two months for several bereaved piggies of mine (none of which wanting live with any other widowed piggy) to be able to date them so they had a say in who they wanted to be with. It was not a great time but I have come out of it with 5 stable and happy long term piggy bonds, so it was well worth the extra wait.
Only if a bereaved piggy is giving up on life and refuses to eat and to interact need you act quickly. Firstly, step in with feeding support; often a bit of syringe feed can trigger the eating reflex again. It is also important that you can exclude a medical problem caused by the extra stress of the loss (especially in frailer and/or older piggies); this is not uncommon.
Real acute pining (choosing to give up on life) is thankfully rare but in this case don't wait to choose, but go for whatever piggy is at hand quickly; they will be in most cases accepted.
Please take the time to read these links here. You will hopefully find them very helpful, both for Truffle and for yourself:
Practical and supportive advice for dealing with loss:
Looking After a Bereaved Guinea Pig
Human Bereavement: Grieving, Coping and Support Links for Guinea Pig Owners and Their Children
All About Syringe Feeding and Medicating Guinea Pigs with Videos and Pictures
Mixed pair specific challenges and bonding advice:
A Closer Look At Pairs (Boars - Sows - Mixed)
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
PS: Unfortunately, the one guinea pig rescue in Vancouver is obviously run to sky high Swiss-style standards which are sadly beyond many Canadian owners but Portland Guinea Pig Rescue in Oregon have a sow spaying policy. I am not sure whether they rehome as far as across the border, though. But if they do, you will adopt a piggy that doesn't come with potential hidden baggage.
PGPR
I hope that this helps you to navigate your most pressing concerns in the coming days.
Please be kind to yourself and give yourself the time to deal with both the shock and the loss; you cannot hurry on the grieving process.
Try to keep in mind that you have bought Buttercup a chance for a good outcome and a comparatively gentle and quick passing that he would have otherwise not have had. Sepsis is not a nice way to die. Feelings of guilt and failure are typical for the onset of the grieving process and a clear expression of your deep sense of love, care and responsibility because you would otherwise not suffer them. They do however not reflect any real failings of yours.