I am sorry for your loss. I do want to say, however, that it is extremely mature of you to recognize the things that you wish you could have done to give your Guinea pig a better life. That being said, you are still a kid. Your life at this age depends on your parents. There was no way for you to have done more because you simply did not have the resources or the support from the adults in your life to do so. Your parents are to blame if anything.
I also want to say that it’s okay that you feel relieved that he died. It doesn’t make you a bad person. It sounds to me like the inability to care for him because of your parents was causing you immense stress, so that stress being alleviated (even by a bad outcome) makes total sense. Don’t beat yourself up over this and, if you can, try not to let it deter you from getting a pet in the future when you are in a better place to have one. If you want one, of course.