You are absolutely right, Tony and Casey met in May about one month before Caylee died. In march she was with Ricardo. Wasn't there something about March and her birthday and St. Patricks day when she couldn't go out because Cindy could not watch Caylee? Wasn't this the time that things were falling apart with Ricardo?
I could be confusing the situation with Ricardo. There was a point in time that she was obsessing over Ricardo and she had the feeling that although Ricardo liked Caylee and allowed her over, he was (unlike Jesse) unwilling to go the distance and become a defacto Dad. She even lied to Jesse and told him he was Caylee's father to make sure that he wouldn't leave her, that is how important the current men in her life are to her.
Things fell apart with Ricardo and then she met Tony. I don't think she was going to take the chance of Tony tiring of dating a single mom. Especially when Tony was not even entertaining an iota of the interaction that Ricardo did.
Sorry if I got any facts screwed up, I have followed from day one but sometimes all of the dates/lies start to blur. lol!
Basically, I have always believed that after Jesse, Caylee simply got in the way of Casey's romantic life. Jesse was the only one willing to become a family. The other two were not. She knew that. And after it went south with Ricardo...she was not letting happen with Tony.
Tony also introduced her to a different kind of social life she never had before. Clubs and paryting. Before that, I think Casey was content with dating a cop and getting married and settling down. (Jesse) and then with Ricardo she was happy to go to house parties, dress up in football jerseys and play beer pong in the basement. Tony's lifestyle required ALOT more going out, new clothes, drinking heavily, promoting DJ's and late hours. Tony was not into Caylee staying over. It required Caylee to be with a babysitter and not having a sleepover in the same bed as her and Tony (like with Ricardo)
This new relationship brought a whole new set of rules and in Casey's self absorbed, manaical world, Caylee fit into none of them so she had to go.
If you look at the progression and differences of the three relationships (Jesse, Ricardo, Tony) I really think it is easy to see, when you are thinking like a sociopath of course.