Do I have depression or not?

I had a pretty rough year and my best friend kind of talked a lot at my back. I didnt find out till much later that a lot of people were bad mouthing me. Its my second year at college and I'm starting to hate that friend more and more. She treats me like i'm so irrelevant. I havent told her that i know she's been talking behind my back. She ignores half the stuff i say. But the thing is she is my cousin too and i cant just pick a fight with her. We used to be really close in school but then we werent in the same place I just feel sad all the time. I used to love to experience new places and new people but now i just hate all of it. I've become anti-social and i dont talk to many people. I have a boyfriend, we've been together over two and a half years. We're in a serious relationship but osmetimes the relationship really stresses me out. When we're having a fight and i feel like he is wrong, sometimes i dont stand up for what i think is right. I used to. Always. But now I've just lost that will to prove other people I'm right when i am. I've lost the will to do so many things. Even when people are joking and i laugh, I'm not truly happy/satisfied/content inside. I just feel really unhappy. A few months back i used to come home and cry every day. But I've gotten more used to sucky college now so it isnt as bad. I remember i used to be miserable. Like really miserable. But i forced myself to indulge in movies and tumblr and things improved. But i feel like I'll never be truly happy. When i'm really sad after a fight or something, I have to control myself a lot and try really hard to stop myself from picking up a blade and cutting. I get a lot of suicidal thoughts all day. Is this depression? Because I have a teacher who is a psychologist and she was explaining depression to a bunch of students the other day and she said that depression caused a person to stay in bed every day and often become unresponsive and lose the will to do anything at all. I cant describe it here enough but what she was saying was very very severe compared to what i was feeling. So then i thought maye i dont have depression like another friend had told me. Could someone explain this to me? Thanks a lot.