Send your peasants to plunder and glory... Let me explain...

You are in the red because of upkeep right? so you need to gain money and reduce upkeep. Don't disband your peasants, send them to the other factions lands, choosing the least defended provinces to atack. You are not gonna keep those provinces, all you want is the loot. Send your peasants deep into enemy lands (you will eventually lose them all but thats not bad) to loot. This will acomplish two things, get you a lot of money (wich you can then use to build a merchant/war fleet, since you cannot afford that right now), plus you will set the AI factions back in the dark ages.
About those rebels... where do you have your king? is he isolated from some parts of his kingdom? He must be in contact with his provinces to control them fully. Anyway it may be to late for preventing revolts and you said you dont have enough troops to deal with them. As Teutonic said, you must regroup somewhere. Be carefull and choose your most valuable provinces, even if this means to lose some of the others. But if you leech on the lands of others (some of Sun Tzu's "Art of War" in this) with your armys of peasants then you will have enough money to build a decent army and retake the rebels, with plans to stay for a long time, this time...
After this, well... you are gonna piss some people off by looting their lands and have a few wars on your hands, but with your new army of Byz troops (built with loot money) there is not much they can do to stop you, since a human player when in crontrol of a major power can take on all the AI factions (not advisable though). But remember that if you decide to have trade, you need to have some peace so, perhaps, while dealing with the rebels, you can try some peace arrangements with good trading nations and blow out of the water all the other ships.
Hope this helps, its hard cause i don't know your exact position...