You need an experienced CPA or attorney. The key question is, did your "marital community" end when you separated, or only when your divorce is final. That will depend on state law, and on the terms of your separation or divorce agreement. We aren't attorneys and can't answer that for you.
On the question of itemized or standard, you are out of luck, at least for the federal return. The rules on the CA state return may be different, but for the federal return, if you are still legally married and file as MFS, if one spouse itemizes, the other spouse must itemize. In other words, it is the itemizing spouse who controls. The only exception is if you provided care in your home for a qualifying child dependent and can file as head of household. In that case, you are not required to itemize even if your spouse itemizes.
