I'm experienced with BDSM and think that when used properly, it can be an excellent way of relieving stress. I'm more into traditional stuff such as flogging, whipping, restraints, etc. and have never given or gotten any punches or slaps around the face. If that's your thing and it doesn't affect your life negatively, then cool. If you'd like ways of experiencing pain and being pushed to your limits without marks on your face, though, you might want to look into receiving pain on the rest of your body. There's a ton of resources online and you can check out your local scene, if you live in a bigger town. PM me if you would like some specific resources, I'm not sure how much I can post here.

Kink isn't a substitute for therapy or even things like meditation, yoga, or breath work - it's a healthy release, but it won't solve your problems. If you're worried about getting "too" into it, that's probably a sign that you need to supplement with other ways of working on your issues.

Boundaries and mutual respect and trust are REALLY important, I can't stress that enough. I can't tell from your story whether the friend you initially played with had a discussion with you about trying these things where you mutually agreed to do them. That talk is very important to have, though - not just once, but often. Either of you is always in their rights to step back and say "Ok, we need to change X."

Good luck - this type of thing can be very fulfilling, though again, it's not a substitute for really working through issues.