My bike is slipping only in highest gear

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.