When is empathy too much?

I have been doing a bit of reflecting as I noticed that I am unbelievably effected by the people around me. I have been feeling extremely exhausted as I have been trying to keep my emotions under control whilst also feeling really affected if my boyfriend or best friend ever seem irritated, upset or annoyed etc (which is difficult because I live with them). I have been looking all over the internet for ways to try and not let other people's moods so deeply affect me. I have always been an extremely empathetic person, however, I feel like it is begining to make me quite unhappy. I am feeling other's pain so much and worrying irrationally for everyone around me that sometimes I feel like I'm always putting my feelings to the back and that is a real strain. But a moment ago something clicked in my head and I had a flashback. Something that my brain had made me forget was that my abusive ex used to be a bit of a lose cannon. I had to constantly watch him out of the corner of my eye to make sure he was ok. The smallest thing could make him fly off which would lead to a multitude of bad scenarios. I would always be checking to try and prevent the worst. It was horrible, I literally couldn't turn off for a moment. I aways had to be watching and checking and I couldn't ever be down or sad or express any unhappiness or it would set something off in him and then it would just go down awful routes. It's crazy cause for months now I have been trying to work out why I am always so on edge round people, constantly evaluating them to check if they're happy and if they're not I swear my chest feels heavy and the anxiety kicks in. It's strange because my brain has blocked out so much so I find myself confused a lot so it's almost relieving when something makes sense... kinda..