Hi, I completely understand how you feel. When I'm down, I crave sex to take my mind off of cutting or scratching, which I have previously considered seriously. But in all honesty, if sex isn't distracting you from living a normal life (doing homework on a daily basis, seeing friends, eating dinner with your family), then you're okay in that situation.
In the case of being suicidal, I say talk to your mother and get help. I did that and tomorrow I'm seeing a therapist. Try to get better before you feel too trapped. Depression is scary, but I know for a fact that anyone can beat it if they find honest support.