Welcome to our forum.
"I don't even know how I'd begin to explore this life if I lose the people I love. I guess I'd just be like your average single person, except with the added baggage that I have to let people that I don't even deeply care about know that being in relationship with me is not traditional."
Some people have had good luck with OKCupid; there's always that to consider. Finding any local poly groups in your area could be helpful. Even here, we have a "Dating & Friendships" board. I don't mean to suggest you should just forget about L and Z; I just mean to be encouraging by saying there are options, even if worse should come to worst.
Even if looking around on our various threads and boards has made you more nervous, you may find that your nerves calm down after awhile. Polyamory is a whole new lifestyle. There's a lot to learn.
I wouldn't rush into anything too fast; also I would advise keeping things at the friendship level with L (as much as possible) unless/until you get around to the difficult poly discussion with Z. I don't see any problem with being honest about your feelings, and if I'm understanding right, you've already told L about your poly inclinations, and are just waiting back for her response.
Just don't feel like you need to make any big decisions before you start to feel a little more comfortable about things. Right now, the status quo is adequate for you. What you need to figure out now is, would you rather travel the "safer path" in life and risk losing out on the possibilities polyamory offers, or would you rather risk losing one or more relationships in order for polyamory to become a possibility for you? It won't be an easy choice, either way, and I just think you should invest considerable thought and time into it before making it.
That's my advice anyway, based on my impressions from reading your posts here. It shouldn't be about "what you're supposed to do;" it should be about "what your heart tells you to do."
Don't know if any of that helps. Anyway, I'm glad you could join our online community.