BIG HUGS
I am ever so sorry. What a horrible shock for you!
Strokes and similar events can sadly happen at any time and at any age. They always happen out of blue so you cannot brace for them. There is nothing you have done wrong; just a genetic time bomb in his body if it is happening at a young age - it's basically a random short straw in the cosmic lottery.
Please be kind with yourself; there is never any obvious cause nor anything you could have done to prevent it. Try to take consolation that he hasn't suffered much and that he has had from you what guinea pigs want from their own life: good care and happy todays. As long as you have given those to Milo you haven't failed him in anyway because the rest is out of your control anyway.
PS: You will find some very practical and helpful advice on what you can do for yourself and any companion of Milo right now and in the coming days via this link here:
Death, Dying, Terminal Illness; Human Grieving and Bereaved Companions: Information and Support for Owners and Their Children
