Please be more forgiving with yourself because you were faced with a task you were not old and mature enough to cope with.
There are no children's pets; there are only family pets. You do not learn responsibility by looking after a pet, you learn responsibility from how you elders live responsibility themselves, support you, control and take over if needed. It's not all your fault; it is the fault of those who have set you up for a fail and who neither did their research nor did give you the ongoing support you needed. Your family has given you basically a breathing toy but has never considered that what they gave you was a pet without preparation or support.
I have grown up with piggies as well - and for the first 10 years it was just one single piggy. But unlike you, we were lucky in that my mother did care for him and ensured that he was cleaned out weekly in turn by one of us children and that we went out regularly during the summer months to pick fresh dandelion from the meadows for his dinner. Very often, and especially in his later years, I had to fill in for my brother, who'd long lost interest. So after Wuschel passed away in old age, it was my sister and I who got a pair of piggies - only that my parents promptly fell into the next trap of giving us a boar and a sow so we had babies twice before my Strolch was neutered (and then promptly put back together with his wife because nobody knew about the need for a waiting list; since the op was rather bungled in hindsight, at least we had a lucky escape in terms of a third back to back pregnancy. But that was all back in the 70ies and 80ies... We all have our journey and our own regrets.
Take it on board, allow it to be your motivation to do better (the same as it has been for me) but do not let your feelings of guilt eat you up and do not fall into a zero tolerance attitude for not allowing yourself to ever make any mistakes again because that way you deny yourself the right grow and develop as a human being. Adulthood is not about being perfect but all about muddling through, stumbling, falling and - most importantly - to get up again and learn more and more deeply through your fall than if you sailed through life serenely without ever putting a foot wrong. A lot of the information I have written for this forum I have learned the hard way - but it has also helped me to see deeper into things and into the connections. Welfare is a moving target; what I am writing now will be outdated again some time in the not very far future. You can only ever aim to do the best you knew at the time.
You may find the first chapter in our parents guide very interesting and thought provoking; it will hopefully also help you to put your current feelings into more of a perspective:
Children And Guinea Pigs - A Guide For Parents
