The honest starting point: a split end cannot be repaired. The strand has separated, and no product rejoins it. Products marketed as "sealing" split ends glue the two halves together temporarily until the next wash.
The only permanent fix is cutting them off. Everything else is prevention.
Why ends split first
The ends are the oldest part of your hair. On shoulder-length hair they are roughly two years old and have been through every wash, brush, dryer and colour since. They are also furthest from the scalp's oil supply.
So the ends are the most exposed and least protected part — which is why damage shows there first.
The five causes
1. Heat styling. The largest single cause. Especially straighteners on dry, unprotected hair.
2. Rough drying. Rubbing with a terry towel roughens the cuticle every time. Squeeze and wrap instead.
3. Brushing wet hair. Wet hair stretches and snaps. Detangle with conditioner in, using a wide-toothed comb, from the ends upward.
4. Chemical processing. Bleaching in particular, and colouring over previously coloured hair.
5. Leaving it too long between trims. A split travels up the strand if left, so waiting turns a small trim into a big one.
What actually helps
Trim regularly. Every eight to twelve weeks, even while growing hair out. Removing 1 cm on schedule beats removing 6 cm once a year, and growing hair longer while ignoring the ends produces long hair that looks thin at the bottom.
Mask weekly, on the lengths and ends.
Leave-in on damp ends after every wash — the ends need what the scalp cannot supply.
Lower the heat, use protection, and take heat-free days.
Sleep on a smoother pillowcase and tie hair loosely — friction overnight is a real contributor.
Do not brush wet.
What products genuinely do
Smooth the cuticle so ends look better and tangle less, reduce further breakage, and make hair easier to handle. That is worthwhile — it slows the damage — but it is maintenance, not repair.
If the ends are very damaged
A larger cut now is more efficient than a year of masks. Damaged ends keep splitting further up, so cutting past the damage saves length in the long run rather than costing it.
For weekly repair, see the hair mask guide.