What personality should I play during child custody dispute?

(California) Hey guys I could really use your help. My exgf and I have a beautiful 2-yr-old son whom I've been taking care of by myself for the last year. Recently I found out she's been sleeping with her coworker for the last four months so we broke up. It is getting pretty ugly because we are talking about the nanny and child-care expenses. She's also taking a two-month leave of absence because she says she's too stressed out to work which means less income for our family. In addition I am her in California on my own and my family is in another country. So her and her mom are somewhat ganging up on me. For a while, I was extremely angry and played a somewhat martyr role. But I realize that it wasn't getting anywhere. I also was sad for a very long time but I think that just made them see me as weak. How should I tailor my behavior in order to get the best negotiation on their side? I could really use the advice. Thanks guys.