Hi and welcome!
I am very sorry for your loss! Eight years is a grand old age to live for a guinea pig. In human terms she would have been well into her nineties. You are welcome to post a tribute to your piggy in our Rainbow Bridge section if you wish to.
Sadly old age doesn't usually come kindly. You haven't killed your piggy as it was simply her time. She had obviously been getting very frail and no longer able to eat and clean herself properly; and she was likely already in organ failure with her body breaking down when you gave her the bath.
Dying is a much more physical process than most people imagine. Drifting gently away in one's sleep is rare! When you have never been confronted with the dying process is can be rather traumatic because you are not able to pick up on the telltale signals and therefore don't know what is actually going on.
The one thing that you can do better with any pets you have in the future is to save up for vet care right from the start and to see a vet if your piggies or any pet is losing weight quickly (weekly weighing throughout life weighing is important) or if you notice signs of bloating/poos not right, and a pet no longer able to clean itself from arthritis - that means that they are not eating correctly and they may be in pain. You would not necessarily have been able to prolong your piggy's life, but you could have made the last days a little easier on her with painkillers and gut stimulants.
But that is an area that you will have more control over as you grow older and it is not something you have to blame yourself for! The fact that your piggy has lived to such a grand age is testament of your good care and the fact that you have these feelings of guilt means that you are a loving and conscientious owner. Pet ownership is a learning curve - we can all do it better next time round as we keep learning and become more aware and sensitive of needs and problems.
We all have feelings of guilt or failure when one of ours dies. That is a normal part of the onset of our grieving process and part of being a responsible and caring owner. We latch onto the areas we feel/know we could and should have done better. Sadly none of us can go back and rewind the clock; all we can do is be more vigilant in whatever bugs us the next time round - and there will hopefully be a next time for you as you go through life!
Give yourself time to grieve. The unconditional love that we share with our pets means that we have to grieve to the same extent as we have loved. It doesn't matter what species; it matters how well we have loved! But without this love in our lives we would be much poorer. And without the grieving we would be less understanding of how precious a gift this unconditional love is and also less understanding of the pain of others when they grapple with their own loss.
Write a diary about your feelings, but also write down all your memories as they come to you and collect pictures. After all those years, you have plenty of precious memories! This will hopefully help you in rebalancing your feelings of guilt with the many more things you have done right. You will also hopefully find that we never fully lose our beloved pets. We will always miss them, but they are also always in our hearts.
You can find out more about the grieving process, what else you can do and where you can look for and find help if you need it in this link here:
Human Bereavement: Grieving, Coping and Support Links for Guinea Pig Owners and Their Children
If your piggy has a companion, here is what you can do for them now and in the coming days:
Looking After a Bereaved Guinea Pig
she was skinny but bloated and i believe she had problems pooping 
it’s been a week..
. I miss her so much 
