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I don't think this makes you the problem. Self harm is an addiction so it's hard to stop, and you have to want to stop before anything will change. That being said, there is comfort in what you know, even if it is unhealthy. That means that even though you know self harm isn't a healthy coping mechanism, it's a safety net and something you already are familiar with. Change is hard. The thing is, you've stopped before so there's nothing to say you won't stop again when you are ready.

Telling your friends and family may be hard, but I think you should do it. I know they will want you to stop, and like I said, that is something you'll do when you are ready. But at least they can support you with some of the things that are going on in your life that are triggering you to self harm in the first place, such as the person in class. I don't think it would ruin your appearance of being strong either. It's even stronger to ask for help when you need it rather than struggling alone and possibly having something bad happen to you.