My son great grandmother has guardianship of him. He is 12. Since me and her grandson split up, she won't allow me to take my son or have me see him. I want to know what my rights are in the state of California or what my chances are to get full legal custody back?

What does the order appointing her guardian say? And of course you can always apply to the court to terminate the guardianship.

Depends on why she was given guardianship. You haven't given us enough info to advise. I would doubt if your rights were terminated, which means you should be able to go to court and petition for custody or, at least, visitation. What your chances are, I can't say.

Do you have an order for visitation that she is violating? If you do then go to court to have it enforced. However, if you don't have anything court ordered then you will most likely need to go to court to establish visitation and/or terminate the guardianship. As mentioned above part of it depends on why she has guardianship and what the original court orders say.