Kayty I'm so sorry to hear of this! I have followed your journey with Hugo the whole time I've been on this forum and it's weird to say but he stole my heart even though I'm on the opposite side of the world.
I can't even imagine how difficult this decision must have been for you to make but I think you did the right thing. Think of it this way: He had an EXTRA year with you whilst you went above and beyond to get him sound again, many would not have done that. You tried everything and rest assured that no stone was left unturned.
Unfortunately, with the length of his racing career (if I remember correctly he raced until he was 7 or 8) the toll taken on his legs and joints would have been severe. Leg injuries can be a dicey game with horses - there is no option for recovery while being fully rested as no horse can ever be made to fully rest one or more legs. If there were such an option, what would the quality of life be? I know you already know this, just wanted to show some support for you during this tough time.
RIP Hugo and hugs to you Kayty.