Had a crusade which went horrifyingly against me for reasons that come down to "because", it seems.
First: The AI knows exactly where my armies are at all times. All attempts to circumvent them failed as they'd adjust course to splatter me.
Second: I can't win major battles. It's literally impossible. I lost a battle where I outnumbered the enemy by 10,000 troops in a Slavic province (Unrelenting doctrine, so HUGE bonuses) with favourable terrain and good unit comp. I get my arse handed to me.
Third: Troops of mine mysteriously vanished. I got a metric fuckload of troops from an event. After the first battle 8,000 of them vanished. The battle was against a tiny army so it wasn't troop losses
Fourth: My allies decided that they needed to go to the opposite end of my country to the invading armies
Bit hard to win when that happens.
I hope it is ok, but I'm going to try to diagnose those points:
First: Spymaster, Diplomat, and Chaplain can be used to scout out the enemy so that you mostly know the same about them as they know about you.
Second: Sometimes, you lose. I'll ask the obvious "How did your commanders match up?". Just keep going I guess when you get that unlucky loss.
Third: Did a war end? You mentioned the battle was against a tiny army, so maybe the event troops disbanded because their target realm collapsed.
Fourth: Allies just suck. You generally need to lead/follow them to where they need to be. They
usuallly will follow your troops if you have some stationed in the next province over. Otherwise they usually roam aimlessly.