BIG HUGS
Strong feelings of failure and guilt are characteristic for the onset of the grieving process since we are wired as humans to reflect everything back onto ourselves. They are however not a sign of you having done anything wrong but of how deeply you care. We all experience them to some degree or other but they tend to be much stronger after a sudden and/or traumatic loss.
Guinea pigs are masters in suppressing pain and illness to an amazing degree. It is all too easy to not pick up on some very subtle hints; and sometimes there aren't any at all until it is too late. It is not your fault and you are not a bad owner. It is one of these things that can happen to any of us; just plain old bad luck.
My Ceri (Welsh for 'Love') was just 520g when she had her emergency operation while I was away in Amsterdam; I hd to give consent to the operation while standing by a canal. The operation was touch and go but she made it through and recovered. That was my 50th birthday... I was very, very lucky that time that a sanctuary owning friend had consented to look after her as a last minute emergency and get her to the vet after the weekend, otherwise I would have lost her.
Ceri had to euthanised 3 months later from a sudden devastating attack of severe bloat; she was just coming up to 5 years. To this day I am wondering whether it was because of how close she had come to dying and whether it had stretched/dmaged her system just too much.
I hope that this will help you. There are always what ifs... sometimes you win and sometimes you lose.
But this has nothing to do with the love and care you give daily to your guinea pigs and which they are measuring a good life by.
Be kind with yourself. Accept that it is OK to not be OK right now. Seek help if you can find some pet bereavement support; it is unfortunately rather patchy in the USA. Take the time to please read our Grieving Guide. It is sensitive but very practical; it will hopefully help you spot some of the unexpected emotional and mind loops we all can get stuck in.
Here is the link again.
Human Bereavement: Grieving, Processing and Support Links for Guinea Pig Owners and Their Children
Diet-wise, your diet is too high in calcium and oxalates (kale, spinach and spring mix should be fed only once weekly, one of them) and too high in sugar and fruit acid, too.
You may find our very detailed and comprehensive food guide helpful:
Long Term Balanced General And Special Needs Guinea Pig Diets
Please do not beat yourself up too much. Pet ownership is a learning process. What counts is not that you stumble; we all do it as adults and we grow as adults by how we deal with our mistakes and not by never setting a foot wrong. What counts is how you get up and what lessons you take away.
You can sadly never pay backward but you can always pay forward and benefit others that come our way. I have lots of regrets, having learned many of my piggy lessons the hard way - sometimes at the price of lives; Ceri is by far not the only one. But I also know that those painful lessons have saved and are still saving hundreds of other piggy lives, including their successors' lives in the Tribe. Ceri was the last of my string of bladder stone piggies that were the result of some diet experimentation when I got the balance exactly wrong for a little while.
But I have taken the lesson on my chin and have had about 70 piggies coming into my life since her passing 10 years ago. However, just one more bladder stone since (which was the result of the fresh food shortages at the start of the pandemic when low calcium/oxalates fresh greens were simply not available. I had to make do with what I could get hold of for 27 piggies for a week and rely on my lawn as much as possible).
But that, I feel, is not the worst tribute I could have made to Ceri. I am however still learning and still tweaking my diet as more research into diet brings new insights but mostly makes that minefield even more complex...
Please do not beat yourself up too much. It is in the nature of mistakes that we cannot spot them ahead, only in hindsight. Your diet selection shows clearly that you have attempted a good diet nd that you are clearly caring deeply about your piggies. Unfortunately you seem to not found access to good information; there is generally a too great liking for fruit in the USA, which is not a food group that guinea pigs have evolved on. But how could you have known otherwise until you stumbled into it?
Try and make Miss Ariel proud by doing something constructive and let her be your own inspiration rather than falling apart over your feelings of guilt and tainting your memory of all the happy days you have given her. that would be the saddest thing and the worst tribute you could make her.
She's a gorgeous little piggy and she deserves to be happy and comfortable. I've given what I can, I hope you reach your goal soon 
