I would definitely ignore an instrument like a Garmin, FitBit, etc. Too many reports of similar stupidity on the part of such gadgets. Diagnosing overreaching, peaking, etc. . . it's getting into an area where pro coaches can disagree. Apps like TrainingPeaks present you with data and the results of well-proven calculations relating to the decay rate of training impulses. But no decisions! Just the data. You have to try to figure out what that means according to your personal experience, and everyone's different, and so the same exact data will mean different things to different people.
Yeah, it's complicated and it takes personal experience to figure out what your response should be to the data presented. You're a lot smarter than a Garmin. Thus we definitely see many riders who say "screw that, I'm going by feel." Which is not a bad response either, but if one is working on a long term project, like prepping for an event, or slowly improving fitness, the apps rule. It took me a couple of years to accumulate enough data and experience with same that I could predict future results of current training just by looking at the numbers. It was worth the effort. I plan my training a couple of weeks in advance by putting various workouts on my calendar and then I see how that goes. Usually I have an exact rate of increase in fitness in mind and I try to put that into practice.
TraningPeaks is so named because the idea is to have some goal in mind and to then engineer a peak fitness for it by doing what I'm talking about. Which means one has to have some idea of what fitness numbers mean and etc., which is the experience part of it. Or one can simply work on getting more fit, but there is an upper limit on what one's fitness numbers can be and on how long one can sustain those numbers, more experience.
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Results matter