Ok, I got Shadowmere, but recently I noticed that when I fast traveled to Falkreath he wasn't

with me. I noticed that about 10 game days ago... Will I ever get him back?

And how. Please help

She has always been rather buggy. Especially when another horse is purchased and after completing certain quests. Try to fast travel back to her pond of origin at the DB entrance.

I did do that. And I did purchase another horse at Lakeview Manor. Should I kill the other horse?

Or what should I do?

I would save before and kill all owned horses. Then return to the DB. There are reports of this working and she was spawned up on the road.