I'm sorry that you're going through a tough time with C. You haven't mentioned in your post if you yourself identify as mono? I got the impression that that was what you were saying.
Having just gotten out of a V relationship, I am familiar with the time issue of quality vs quanity/primary/secondary caboodle. I've heard a lot of other people talk about it. When you're the live in partner, you get stuck with the everyday stuff like paying bills and doing the laundry. You tend to spend a lot of time together but much of it doing your own thing in the same house. Whereas time with the non-live-in person tends to be purposefully, recreational and free of the mundane distractions. I think that one of the best things to do for that specifically is to be conscious of having quality, focused time with all partners. I would pick a couple hours of focused quality time over a weekend of being together doing our own things any day.
Be careful of assuming that the only two options are that you're defective or C is bad at choosing partners. It sounds like one or more of you haven't been doing poly for that long. And that you and C are a relatively newer couple. It's ok if making this work just needs a lot more negotiating, self-examination and patience. And it's ok if this relationship isn't meeting your needs. But it sounds like you're wanting to keep trying. Yes, it's totally reasonable to want to feel valued. That can, however, be a vague request. I would try to think of some concrete ways that C could show how much he cares about you and values you. And if his girlfriends see you as an obstacle that would be a red flag to me. ie Cowgirl! Nothing wrong with making sure his girlfriend/s understand what your goals are as a couple practicing poly. Have you had much chance to talk with Misty one-on-one? Oh, if you look up miss poly manners, she did a really awesome segment on the polyweekly podcast regarding how to reach out to a new partner. It has some good ideas.