Like most people in history who decide to rule the world, you've come undone by lack of preparation and lack of resources. One does not simply walk out their front door, declare a kingdom, and not have to do hard work.
My advice...
- Get your trade stats up - so you have passive reputation gain. You need a lot of money to grow a kingdom by bribing lords to join you and after a slow start, the trade tree is great for this.
- Get your charm skills up so lords like you - especially under 1.5.7 where they're less likely to attack you if they like you - This is easy to do by releasing every lord you capture after battle.
- Get your leadership stats up - I can't specifically say why, but now that my leadership is over 200, lords are falling over themselves to be my friend. You can do this by making a second or 3rd party and keeping them permanently in an army with you. I currently have 3 companion parties permanently in an army with me. They might not be completing quests and the like, but the leadership gains are great. As a bonus, you get influence by having your parties in an army - even if it's you army.
Then it's just a case of tracking down clan leaders. The ones with no fiefs are cheapest, followed by those who only own castles, followed by those who own cities. Prepare to spend millions for cities (which is why you start by building your trade skills up)