my boyfriend hits me.

my boyfriend of 4 years hits me, I tried to leave last night and he threatens to take my kids from me, because I had a anxiety attack and he accuses me of being crazy it only happened one time, and now he says he can take them and I can't fight back because I don't work full time. I want to leave but I can't risk losing my kids they are all I have. he's the dad of two of them not my oldest, I had pictures form the time before but he deleted them from my phone.