You know, you've gotten a lot of feedback on what you should or shouldn't have done.
I'm going to do something different. I'm going to tell you that I know just how you feel, and I empathize. I don't think anybody would argue that I got my DM cert too soon; I had 800 dives, Full Cave and a tech cert before I began it. I had lots of experience in diving in low visibility in Puget Sound. And doing OW dives with students sometimes scares me half to death. It is one thing to maintain your own composure and manage your own issues in low viz and dark and depth, and quite another to deal with a student who isn't managing himself, may be frightened, and could be close to panic. No one who leads guided dives in warm, clear water can understand the sheer terror of being unable to see the student you're responsible for, because he's just kicked up a siltstorm that's rendered him completely invisible, even though he's less than an arm's length away.
There are days when I wonder why I DM -- and then I remember I really didn't have much of a choice, since the instructor I work for is my husband.

I love pool work, and have very ambivalent feelings about dives in OW. When they go well, they're fantastic fun, and when they don't, they're stressful or downright scary. (One of my most horrible memories was riding a corking student from 50 feet to the surface, trying to slow her ascent, and just desperately hoping she was still remembering to breathe.)
One DOES learn to be assertive. Last Friday, we began an OW1 dive, and had two students, with Peter on one side of the pair and me on the other. When we descended, I couldn't see the student on the other side of mine; heck, I couldn't see my own student's right arm! I immediately gave my student the thumb, and we surfaced and waited for Peter. When he came up with his student, I told him I thought doing two students at the same time in that viz was unsafe, and I wasn't willing to do it. He agreed, and did Dive 1 with each student individually, in touch contact; he then canceled the second dive for both.
Don't do things you don't think are safe. If you need to surface a student who is low on gas, do it, and deal with the fallout afterward. If you're wrong, you may have some consequences to face, but they can't be as bad as the consequences you'll face if you're right and you don't listen to yourself.