My guinea pig had a surgery for bladder stone yesterday and he passed away. I’m really breaking down and can’t function. He was my little baby.
How was that possible. I read so many success stories

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Hi
BIG HUGS
I am very sorry.
Sadly, there is no such thing as a 'totally safe' operation and you can never tell whether you are one of those ending up with a short straw. I have lost several piggies over the years where the operation itself was successful but there were unknown underlying conditions which meant that their heart was giving out on the operating table or they passed away shortly after the operation. It can happen even with the best operating vets.
It is even harder to commit to the same necessary operation again down the line but thankfully, those have always been successful for me.
Did your vet say where the stone was located and whether it or any crystals had embedded in the bladder wall? Boars also have an awkward inglenook in their urethra in which stones can fetch up and which are a much more high risk make-or-break operation that often doesn't come off. A bladder stone op can get rather complicated very quickly if it is not just a single loose sitting stone. The longer a piggy is under GA, the higher the risk of complications stemming from the anaesthesia.
You are of course distraught - we all are in this situation - but there is no reliable medical cure for bladder stones, so if you hadn't committed to the operation, your boy would have instead either have died a slow and very painful death or a truly agonising but much faster death if the stone had at some point blocked the flow of urine so it would have fatally backed up into the kidneys.
Please try to take heart in that you have absolutely done the right thing for your boy to spare him much worse.
You have not failed him! The outcome is out of your control but you leaving him to suffer for the - rather short - remainder of his life would have frankly been much worse for him. You have bought your beloved boy a vital chance for a happier, longer life that he would have otherwise not have had; it was just bad luck for him and you that it hasn't worked out. But you have still spared him days or weeks of suffering in pain and that counts for a real lot.
Give yourself time to grieve and be kind with yourself. Try not to be too hard on yourself and try not to be angry with the vet team. They always try their best and any fatal outcome is really tough and morale breaking for them, too.
If you really struggle with your various strong emotions and mood swings, please seek trained help, like the various free of charge pet bereavement services that the Blue Cross offers for UK residents. The best thing you can do is talk. If you can't find anybody, then try to write a diary about your feelings and thoughts.
Pet bereavement and pet loss
You can find more helpful advice and information to make sense of your traumatic experience and your grieving process, which can take some unexpected turns, in this link here:
Human Bereavement: Grieving, Processing and Support Links for Guinea Pig Owners and Their Children
If you have any companions or other piggies, please make sure that they are OK and able to hang in there right now and for the coming days until you are over the worst of your own storm:
Looking After a Bereaved Guinea Pig
My thoughts are with you.