Just couldn't bypass this one without saying how sorry I am. It isn't ever easy making the final decision. If they live to 25 even, it still isn't easy (we lost one this age at my yard). I've lost one at 10yo, same as you OP, and it really sucks.
There is just not a "good" time to lose a heart-horse, it isn't easy whatever time of the year it is.
So very very sorry for your loss. Thinking of you X