Ok, I am definitely not a gushy, emotional person. So keep that in mind when reading my response. Initially, I want to say, "get over it." But I know that many people think that is harsh so I will expand on that:
The thing is - it's done. Done, over and no changing it. So I have always been of the opinion that I am not going to expend energy and resources on something that can't be changed.
Now, you need to come to terms with that. Remember, the horse really didn't have any concept of goodbye or whether you said "bye, grumpy" or "see you tomorrow, love." The emotional chagrin and guilt is self-imposed. You have to realize and accept that at any given moment we all make decisions the very best way that we can given the facts at the moment, the emotions at the moment, the skills at the moment, the colour of the sky and whatever other thing was going on in your world then. Those decisions are made by you, by the vet, by any other person that interacted with the horse and so on.
So, remember AT THE TIME, the very best decision that could have been made, was made FOR THE BENEFIT OF YOUR HORSE. If it would truly have benefited your horse to wait for you to appear to tell him "I love you" that he wouldn't have understood anyway, then that is what probably would have been done.
Don't make yourself feel bad or guilty over the past by dwelling on it. As already posted, remember the fun stuff.
Am I going to get shunned by everyone now for saying this?