Had exactly the same problem. At some point I had 9 nations against me without attacking anyone. I had to establish a very defensive strategy, not expanding for a long time and making some sacrifices :
Block access to the Pyrénées for Portugal and Spain, retraining in Toulouse when necessary. Two armies and alliance with the moors (doing poorly thought).
Giving up my crusader states to the pope : Byzantium, Egypt and the Turks all agreed to a ceasefire the next turn (I kept Cyprus and Rhodes on the other hand, they allowed me to get some trade income without bordering anyone there).
Get Caen asap. Once taken England should also agree to a ceasefire the same turn. After that, english just sat on their island for the rest of the game. Best trade partners in the game. I even think attacking normandy asap should be the best move (well i'll try next time, they ended up betraying the alliance anyway).
On east I just defended the cities the best I could, never breaking siege unless its absolutely necessary. This tactic made Empire, danes and Milan to get excommunicated frequently (keep good relationships with the pope, easy one if you dont initiate the fights and go crusading).
Obviously this allows whole west part to be defended poorly and used to replenish east front. Crusading against HRE and Danes got me a few cities (Staufen and Bern) but made the war with HRE last for more than 200 turns so next time I will probably go south first and crush those cheap milanese militias.

I really really hope this bordering-stubborn-stupidity diplomacy is at least moddable. Portugal and Milan really did make suicide on me, never being able to send sufficient forces (or the whole family when they did, bad idea if you ask me).