I'm a new diver as well, although I've always been very comfortable in the water (and originally did dive training way back in 1982, but just didn't have the opportunity and finances to actually get back to getting certified until now!) We did our first 3 skills dives in Vortex Springs, FL but the hotel I was in was right on a local highway and I had gotten maybe 5 hours of sleep the night before. Just a little tired for that first day but not a problem. The next day we were to dive in the Gulf of Mexico, meeting up an hours drive from where we were staying at 5 o'clock in the morning... I barely got 2 and half hours of sleep that night so I showed up at the dive shop and told my instructor that I wouldn't be diving as I was far too sleep deprived (a situation akin to be "buzzed", if not drunk) and even had a bit of upper sinus stuffiness. I wasn't nervous about the dive or my skills or anything, and I'm sure it was a fairly safe environment as there were many, very attentive, dive instructors and DMs watching over us, but I still felt it better to cancel.
I was quite surprised that this instructor, a very experienced, accomplished technical and cave diver with many thousands of dives over decades of experience who often mentioned during training the adage that anyone may cancel a dive for any reason, no questions asked (other than, perhaps, "Are you ok?" after surfacing) immediately responded with "But you HAVE to dive! You won't get your certification without getting one more dive in." I told him the situation and he immediately agreed that if I wasn't comfortable I shouldn't dive, but he very much surprised me with that response, I will admit. I'll be finishing my certification with another dive this next weekend, but I'm convinced it was still the right thing to do. Even if the situation was one being surrounded by many dive instructors and dive masters watching over all of us I would hate to be the one that just did that one stupid mistake because I just wasn't thinking straight and suddenly everyone else's final certification dives gets cancelled and they all have to do it again some other time because some potential emergency that involved needing to get me evacuated. It wouldn't have been good for anyone if I had made that decision to dive and something had happened.
If it's not obvious I very much agree that it's better to call a dive any time you feel it's "not right" than to take the risk that something might go wrong. It's one thing to push through anxiety and just general anxiousness to dive, but too many close-call and really bad dive stories start with "Something just didn't feel right", it seems to me. Better safe than sorry, especially when a bad outcome could affect a whole lot more folks than just you.