Help...

I’m a new mom. My son is 3 and a half months old. I’m bipolar. Plus I developed postpartum depression. I’m medicated for the depression but because of my bipolar the anti depressants make me angry. I can control that. But my doctor told me taking lithium while breast feeding could potentially hurt my son. I desperately want to breast feed but I’m losing control of my emotions. Not with my son but at work, mostly.. and my relationship with my SO is suffering immensely too. I know it’s ok to give my son formula. My mental health is important for him too and I know that..... but I really need to be told it’s ok.... I feel like such a failure. My son needs me to feed him and I’m thinking about giving up because I just want to be happy. How selfish am I?