I'm afraid your "real" issue would be hard to pin down. Your post illustrated how your life is filled with crisscrossing schedules and complications now-a-days. In immediate terms your dilemma is how/when to fulfill your desires for lovemaking with S. Taking a step back, a wider dilemma comes into view; that is, how do you time/handle/satisfy a whole lot of romantically-entwined relationships at once?
It is, in essence, the cross that every polyamorist must bear. Just having two romantic partners can be a coordination nightmare at times. And for every additional romantic partner beyond that (regardless of who's connected to whom), the situation gets geometrically more challenging. Part of the dilemma, alas, is deciding how large and complex of an intimate network you can realistically manage to be a part of. If there gets to be too many people for you to juggle, you have to consider subtracting a partner or two, or even removing yourself from the network if no one else wants to do so.
On the other hand, if your heart is set on keeping all the people in the network that you have (or even on adding more people), then you'll probably have to resign yourself to some scenarios that aren't very ideal for you. At times that may very well mean putting sex off when you greatly desire it, as well as "forcing yourself into the mood" when it doesn't strike you spontaneously because, well, Monday and Thursday is all you really comfortably have for that sort of thing with S.
If you stay the course and ride the waves, you'll probably discover tips and tricks along the way that will help take off the edges. And you'll find that you'll get more used to things that feel awkward now. Eventually and with Lady Luck's blessing, your intimate network will find its "sweet spot" where things seem to be running smoothly and everyone's wants and needs are astonishingly met to everyone's reasonable satisfaction.
You were probably hoping for more specific feedback, but the problem there is that all people are so very unique that it's basically impossible to determine the solution to one network's problem/s, especially when the equation's as loaded with variables as this one is. Being closer to the situation than any other Polyamory.com member, you're left with the unenviable task of tinkering with the problem and finding the solution that's right for you.
Me, personally, I just got used to having sex less often, to scheduling it, and even to "enjoying" a reduced libido. Which is okay because: the sex the lady and I have is still great (even if it takes us a few suspenseful minutes to warm up). And even if we only have it once a week (sometimes even skipping a week to work around some unusual circumstance), we still feel satisfied with what we've got. I suppose it helped that our "NRE wore away" to a large extent.
But y'see, that's *our* solution. That's what works for us. It's not necessarily gonna be what works for you.
You may be due for a sit-down with (at least) S and C to discuss the situation. Hopefully y'all are having regular sit-downs anyway; newly-forming poly set-ups especially tend to need that. But I'm sensing some tension between you and C that, if ironed out, might yield some more lovemaking opportunities for you and S (and perhaps C and S as well).
As for being a glass-half-empty guy, might I suggest just adopting a more-optimistic view of yourself in the mirror? After all, if no one examined the problems at hand, then the problems would never get analyzed and solved. I suppose some problems will go away by themselves after a spell, but many problems will wait on our conscious intervention and therefore require us to be conscious of, and pay attention to, them. In a word: *someone* ought to be minding the empty part of the glass.
If you were all happy-go-lucky about everything and said, "Oh well, never mind, all will be well," then you probably wouldn't even be here (on this forum) looking for answers. So while adopting some optimism would probably do you some good healthwise (and sanitywise), don't be too quick to go overboard in that endeavor. Reality is a mixed bag (a glass that's half full as well as half empty), so you need to appreciate, be aware of, and contemplate both the good and the bad.
Other than that, the best I can probably do for you is just continue reading your posts, draw new insights/information from them, and post any new ideas I can come up with for you as a result.
So perhaps, keep the conversations going, is my more short-winded advice?
Love means never having to say, "Put down that meat cleaver!"