You might not know what it is, but there is some reason why you want to cause yourself pain. Maybe you want to be in control of the pain, because if you don't have control, pain of some kind would be too much for you. You say that you aren't all that depressed, but that you are constantly low. I wonder if you are using SI and alcohol in order to keep the low mood from being worse. I'm not a big fan of meds, but constantly feeling low sounds to me like dysthymic disorder, which is the kind of depression that isn't necessarily really severe but it is persistant. I've had it for years and years, and it's miserable, and it just gets tiring. Lots of things can contribute to it - genetics, lifestyle, stuff from your past that hurt you. But it doesn't have to always be that way. Meds might be worth it if they can help you not to feel low constantly. Or you could try something like St. John's Wort, which is what I'm doing. I like SJW because I don't have any side effects, and I can control it, and could stop if I wanted to without withdrawal effects (except that I do relapse if I stop taking it). It isn't a complete cure, but it makes a big difference.

There is a reason for all of this, and you don't have to just live with it. It can change.