Oh Caroline... sounds like focusing on the 'numbers' is sucking the fun out of it.

A quick note on Heart Rate. For my most recent activity (25km) the garmin watch gave me the following chart:

And yet when it uploaded that data to Strava, the strava one was this:

Garmin - 67% at Zone 4, 32% at Zone 3
Strava - 25% at Zone 3, 75% at Zone 2

That is a huge difference in the interpretation of my heart rate.

Obviously its because they use different numbers to define each zone... but if I go on how I feel, then I felt more like the strava interpretation. I wasn't 'plodding' but I certainly wasn't pushing hard. For your sessions last week, did it feel as hard as the garmin watch said? When it said 'maximum', did you feel like you were going all out?

The other thing to consider (which you have already alluded to) is that LOTS of things can affect your heart rate: heat, stress, flat vs hilly terrain, menstrual cycle phase, lack of sleep, illness etc etc etc.

If any one of those raises your heart rate by an extra 5bpm, that can be enough to move you from Zone 2 into Zone 3

As for Cadence, again I have to ask what is so special about 180? Surely the focus should be to increase cadence... so if you used to average 170, and now you're averaging 175, that's a win in my book. Getting frustrated at not reaching the magic 180 sounds the opposite of keeping it 'Easy'