As a swadian vassal you should not become a rebel, this should only happen when you are neutral and conquer a castle or village

Sounds like a bug.
Now that you are a rebel you have two options:
- try to conquer the 5 kingdoms alone (or survive at least)
- support one of the claimants (anyone of the five) - that way you will become the rebel of <based of claimant> faction, lords can join you and if you win the claimant will be the new king.
To my knowledge it is not possible to join an existing faction as long as you have castle or city. Both of the above have been handled in different threads, I myself advice you to support a claimant.
Being a rebel you should improve your defences. The lords like to attack the weakest garrison first but they disregard troop quality and just go by numbers. To keep your holdings safe you just need hundreds of recruits.
If you support a claimant try to find one of the faction with the most lords that like you - these can be persuaded to join the rebellion and help you in the fights against the old king.