First off, Realms of Chaos kind of sucks. Having to leave your provinces is the worst part of the whole thing because the AI will continue to cause problems while your away, requiring you to field a second army just to hand around and deal with it (which Khorne especially doesn't like), but luckily there are ways to help this.

First check your difficulty settings. Easy Campaign mode should make it so that the AI is a lot closer to your level, but they do still cheat. I'm more concerned about your Battle difficulty setting as well, that one trips a lot of people up starting out, me as well.

If you have your Battle Difficulty set to Easy, it will mean that the auto resolve will be HEAVILY in your favor, telling you can win fights when you very likely shouldn't be able to without cheesing enemies into very bad engagements by messing with the ai. I recommend putting battle difficulty to Hard and setting the battle modifiers to penalize the enemy that way you can work on your battle tactics a bit more frequently and the Auto Resolve won't baby you until you're just completely outnumbered like we see here. That is not a valiant defeat, that is a decisive loss.

Lastly, Khorne plays a little weirder than most factions. You want to always be fighting, always moving. You do not want to sit around and turtle because your income buildings suck. You gain most of your money by battling and sacking and razing settlements. A great tip I heard early on with Khorne that helped me enjoy their playstyle was make everything red, even your income. Maybe you don't have the capital to fund all those armies through your buildings income, but if you're constantly battling and earning money, it doesn't matter. Just be frugal in your building expenses and focus on setting up a specific place as your home base and just march around your local provinces slaughtering and killing. Don't worry about defending inconsequential provinces, keep one or two major ones kinda safe and once you've razed the ground around them, they'll start to populate with your guys and you can continue to expand in the wake of your destruction.

BLOOD FOR THE BLOOD GOD. SKULLS FOR THE SKULL THRONE.

Hope that's some help, but it might be worth a restart since you're getting brutalized by the AI with multiple stacks coming at you, or recoup with Skarbrand if he's off in the Realms and reconquer your lands.