I used to ride my bike in highest gear all the time, even uphill, and didn't shift down unless i absolutely needed to. Then my highest gear started slipping on uphill and even on straight road if i pedal a bit harder. All the other gears work fine, and i'm currently riding in the second highest gear no problem
The smaller sprockets can wear quickly because they have so few teeth. You can try getting single replacement sprockets, but generally you’d replace the whole cassette. Be aware that you’ll probably also have to replace your chain because you should only use a somewhat new chain on a new cassette.
For the future you should:
- Get bigger chainrings if possible so you don’t have to ride on the smaller sprockets all the time. Also gives you harder gears, since you seem to like them and have few hills.
- Replace the chain early enough. A badly worn chain will speed up wear of the cassette and chainrings. Get a chain wear gauge (they are cheap) and replace the chain at ~0.75% elongation.
- Increase your cadence. Riding in hard gears at low cadence can tire your muscles quicker and can be bad for your knees.