HUGS
I am so sorry that Ohmi's loss is coming with bigger long term changes for you and is making it somewhat harder that way.
Please be aware that soul-searching and feelings of guilt are typical for the onset of any loving owner's grieving process but that this is naturally stronger after a somewhat traumatic death. Our grieving guide explains about that, as well as the next stages to help you brace for the inevitable adrenaline hangover which will add some rather physical experiences of feeling very 'blue' and down indeed.
Losing Pieta will also mean that you are going through a bit of a grieving process for him, too; and that this is going to make the coming weeks while you get used to not having guinea pigs around in all the little and unthinking ways even harder on you because of the jarring reminders. Please be kind with yourself and seek help for pet bereavement if needed (you should find what there is available in your country when googling online). The guide link below also contains other practical ways of how you can help yourself with the processing of your grief.
Human Bereavement: Grieving, Coping and Support Links for Guinea Pig Owners and Their Children
You HAVE made the right decision for Ohmi. He may have lived a little longer but he would have done so in increasing and even greater pain since bladder stones do not just disappear on their own; they grow and at the worst end up in the urethra where they can cause even worse havoc and can kill outright by blocking the urine flow, which is extremely painful. One a piggy is starting to lose weight because of stones, you are dealing with a problem that is not solving itself on its own and that needs intervention because otherwise it is a very agonising death.
Ohmi's body will closed down post-op because of the GA drugs but I can assure you that he would have been free of the excruciating pain from the stones in his last hours (which stops instantly as soon as they are out and that the pain in the bladder walls from the stones is much greater than the pain from the vet's incision) and that he was still protected by the strong painkillers from the operation when his body went into organ failure. I hope that this will help you.