Christina - One of the best ways of learning to understand and analyse your own work. Good on you for being willing and able to do that. It means you'll be much more aware of these issues the next time you try something like this.
For me, you have identified the key issues yourself.
In terms of sharpening:
"Once in photoshop elements I tried to select around just the horse to sharpen just the horse but I could not manage to do it with any finesse (the horses mane is a challenge to select) so instead I used mask sharpening on the entire photo ... "
I recognise what you are saying about trying to select in order to selectively sharpen. The other option is to not to try and select an area, but to use the opacity slider on the sharpening tool. For something like the mane, you can try turning the opacity way down and then gently brush over that area a few times to build up the sharpening effect. Remember that less is often more and in a picture such as those you have here the edges of the mane would not necessarily have to be absolutely pin-sharp, so long as the main part of the body and especially the eyes, are.
In terms of your wish to have more clarity, I wonder if you mean that you want more 'pop' in the picture; i.e. somethign that maybe doesn't look so flat? If so, the answer two that one is Local Contrast Enhancement (LCE). Again, by selectively applying LCE in the same way as you selectively apply sharpening, you would get a lot more clarity/'pop' into the coat. You can read about LCE in
this CiC tutorial