One of the many things that fitness has taught me is that I often get the best results when I'm not great at something. When I can't do a certain move or type of workout perfectly and with ease, that's when my body and mind are being challenged in a way that makes them stronger and better. So I've learned to tune in and pay attention to some counterintuitive cues: for one, when what used to feel super hard begins to feel surprisingly less so, it's time to change it up, because my body has adapted to that challenge (don't forget to revel in that accomplishment, btw!). That doesn't mean you should abandon that exercise entirely; just change something about it - the order, number of reps, length of sprint or recovery intervals - so that your body is surprised by it again.
Another counterintuitive cue I've come to rely on is that the things that cause me discomfort (not to be confused with pain) are the ones likely to bring me the most growth. This often happens to me with strength training - I've got my tried and true moves that I base a lot of my workouts around, and like a good strength-trainer, I vary reps, load, etc. - but sometimes I'll be introduced to a move that I don't particularly like at first. Not because it doesn't feel effective, but because it just feels awkward and I can't do it with ease... it doesn't feel familiar or comfortable. The upside is that I have to focus more intensely and get in touch with different parts of myself than those I'm used to calling on, which in turn makes me more well-rounded and gives my often-used muscles a break. So I've developed an affinity for the awkward and challenging, since I now know the rewards they contain if I can sidestep my ego enough to reach them.
In the new year, I encourage you to embrace - and even seek out - opportunities to feel somewhat clueless and uncomfortable. And when you feel self-conscious or even a bit silly, congratulations - you're on the fast track to progress!