first of all to mkae peace with the other factions, you need enough right to rule.
if you don't have enough, they will not even listen to you and keep attacking you till your kingdom lies down in ruins.
so send you companions out to gather more of it.
think that, when you reached about 40~50 rtr the factions would accept peace, if you pay them.
and of cause the faction you got your castle from, wants to have it back...
sometimes if you have a town and at least one vassal, the guildmaster of the town could give you the quest to stop the lords on each side which wants to continue the war.
i'm sure you know that kind of quest, so if you got such a quest, DO IT!
it helps very well
that is, why it's very hard to set up your own kingdom if you haven't got enough gold to pay huge armies, or haven't worked as a vassal to an other kingdom
i do not guarantee any of these points will help