Can you still be a paramedic if you used to self harm?

Well it depends on what the rules are in your country/state, but I would say yes, you can! If you haven't done it in a long time, then why not? Worst they can ask of you is to get a psychiatrist to declare you healthy.

Only problem I can see is that it is a very challenging job, you see many bad situations, and if you are prone to psychological problems (self injury is a psychological problem) then it might be more than you can handle.

It is a though, but very enjoyable field to work in!