If a lot of people think you are a cold, selfish person, then I think the problem may extend beyond your ability to tell the difference between having empathy and enabling self-harming thoughts. It's really not up to us to judge someone else's reason for seeking emotional support, or to measure their pain. It's not our place to diagnose. For me the line is if someone is using or abusing me - then I lose empathy. But there are a lot of factors. You might want to talk to a professional if this is affecting your relationships, which it sounds like it is.