If you can maintain 18 km/h into a wind ... it isn't much of a wind. Keep riding, you're fine.

If you can maintain 15 km/h into a wind ... there's a bit of a breeze. Shift into an easier gear and relax. It's like a very gradual hill.

If you are struggling to maintain 10 km/h into a wind ... that's a pretty strong wind. That's a character and leg-strength-building challenge! Shift into an easier gear, relax, and try to spin as much as you can. Anytime there's a lull in the wind, push as hard as you can, and when you get back into the wind again, relax again and spin.

Don't push too hard when you're actually riding into the wind, push when the wind lets up, like when the road turns a bit, or when there are some trees blocking the wind. Lots of people I've ridden with do it the opposite way ... they push hard into the wind and exhaust themselves, and then relax when the wind lets up a bit. But you get further and feel better if you just relax and spin into the wind, and then push it each time the wind eases up. As slow as I am, I've dropped stronger riders that way (much to my surprise!).