Do not blame yourself!! This is what my mother did when out cat of 15 years (a family member, a person without a pet could not understand) was
diagnosed with womb puss. She did everything she could, staid with the cat for nights, we gave her medicine every 3 hours. She volmited, she wasn´t feeling any better, it was hell. Still to this day (this happened 4 years ago) she blames herself for "trying too hard" or "I can´t believe I put her through that, I should have just let her go". This is what parents do, you were the mama of this horse, you couldn´t let him go without a fight.