similar thing just happened to me. I went got the chance for power thing, but waited I guess too long to attack (?) and when I did it said I did not have enough support. couple years later I got the chance for power again, all the romans turned on me, and I took over arrertrium, arrminium, rome, capua. I got a ceasefire with Scpii in order kill off the Julii, then the scpii attack me and it said i was neutral are you sure you want to attack them? and so I think it is a problem with roman => roman diplomacy not something else. I think yer screwed, since I had to wait until the people approved of me again, and since the senate is dead that won't happen. Good luck though!