Which habit was doing the most damage in hindsight — heat, the towel, or brushing it wet?

Wet brushing with a paddle brush, by a distance, and it's the one you described as unavoidable.

Hair swells when it's wet and the internal structure is at its weakest, so a brush that drags a whole sheet of hair from the roots down is applying tension exactly when the fibre can least take it. A paddle brush is the worst tool for it because the pins are set in a flat bed and can't flex around a knot — it pulls the knot instead of passing through it.

I switched to a flexible detangling brush, sections of about four, starting at the ends and working up, always with conditioner still in. The breakage band grew out over about eight months and hasn't come back. I still blow dry.

I'd nominate the heat, and specifically heat with nothing on the hair.

The reason it beats the towel for me is frequency times temperature: three times a week at the highest setting is roughly 150 sessions a year on the same mid-lengths, and the mid-lengths are the oldest hair on your head that isn't the ends. Dropping to medium heat and finishing cold cost me maybe four extra minutes per dry.

If you only change one thing this week, change the setting. It's free and you find out fast.

Going to disagree with both of the above and say it's the towel plus whatever you tie it up with.

Rough drying with a terry towel is thousands of tiny hooks catching on a raised cuticle, and the classic towel turban puts the whole weight of wet hair on the hairline. Add a tight elastic in the same spot every day and you have a mechanical explanation for a band of breakage at one height, which is what she's describing — heat damage is usually more diffuse and worst at the ends.

Microfibre or an old cotton t-shirt, squeeze rather than rub, and change where the tie sits. Cost me nothing.

Hair is dead, so nothing repairs — you're only ever choosing which damage to stop adding. Rank your habits by how often they happen, not by how harsh they feel. Daily wet brushing beats three blow dries a week on that count every time.

I cut heat out entirely for four months to prove a point to myself and my hair looked no better. What did change things was replacing a cotton pillowcase with a smooth one and stopping sleeping with it loose and getting wrapped around itself.

So my honest experience is that the overnight friction was doing more than the dryer, and I gave up three months of not-blow-drying to learn it. Change one thing at a time like you said — I changed two and had to redo it.