After winning the short game two times i started a long game and did it too well...now i have the following problem

every few seasons the people of my clan grow angry and restless because the population is to high and they demand a split. I can only choose the "Split in bad terms" because my clan is allready part of a kingdom and all neighbors are allies.

Does anybody know a way to stop this endless cycle without giving up land?

I had to split maybe 3 times during the whole long game on my latest complete playthrough. If you have to split every few seasons, there is definitely something off.

Personally, I try not to grow to quickly (no child-magic if I'm above 1000 clansmembers, for example), and when things go well I focus on getting more cows rather than more people.

Unless your problem is you refuse to split, and your angry clansmen come with the same grievance every year. Then I'm afraid you might have to split. It's kinda part of the natural lifecycle of a succesful clan. That's the reason why I try to be on the border of the tribe, rather than in the middle. Having a nearby hostile punching bag is always convenient.

I'm kinda curious, when you split you clan do you instantly have an alliance with them? Because last game when my clan split (which may have been what ruined me because they took like 600 of my cows with them, combined with the fact that I rushed into a tribe too quickly while not having good relations with most of my neighbors) and went into unexplored territory I though that I sent them an emissary asking for an alliance I never got the alliance negotiation event. Although it's possible that they got stoped and I forgot about it, next thing that I knew I had alliance with them!

Also for the last few years now in my current game, literally every tribe that I go to hoping to trade my trade goods or horses with them for cattle has said that they cannot trade with me because they need all of their cattle. What's up with that? Surely they cannot all be low on cattle!

I’m pretty sure how you handle the split determines their relationship.

As for buying cattle, that sounds a bit like one of the tweaks to the economics in 2.0.