Sandra,how awful, I feel so bad for you. I know how much fun you've had with your horses. (Eight of the horses I used to ride, in lessons and as "leased horses" have passed on over the 12 years that I've been taking lessons. But I didn't have to make the decision to put them to sleep. The owners were left with that burden). You'll see bits of his personality in the horses that come into your life afterwards. All you can know is that you gave him a good life, cared for him to the best of your ability, and in the end did what was humane. Cold comfort I know, but for now keep telling yourself that. Take extra care of your other horse, he will miss his friend. Give him some carrots and a hug from me.
Time doesn't heal all wounds, love does. I still miss the cat that I had for 13 years and my dog that I had to have put to sleep because she ruptured a disc in her back. Our setter had cancer and she was put to sleep when she was unable to enjoy her life. I have two more cats and a dog, (who is snoring on the floor right now). They fill the space left by the others.