Nicolas Cage's ex, Christina Fulton, is devastated -- saying being forced to take legal action after her son Weston's alleged attack on her has left her completely heartbroken as a mother.

We spoke to Christina before she entered the courthouse Monday, and her pain was palpable -- saying she’s been left to fight this legal battle alone, abandoned by the LAPD, who she claims left her bleeding on the ground after the attack last April.

Christina also told us Nicolas has completely cut her off -- blocking her and making the deep divide between them painfully clear.

You’ve got to watch the whole clip -- Christina is incredibly emotional, saying she loves her son, but at this point, it’s all about tough love because he struggles with mental health issues.

As for suing Nicolas, she makes it clear she’s holding everyone accountable for what she believes happened to her son -- adding the tension between both of them is far from over ... after all, she’s in court.

Christina is suing Weston for assault, battery, negligence and infliction of emotional distress -- but she also filed a lawsuit against Nicolas for fueling their son's violent tendencies.