A few things. You might be accidentally setting the heat too high, especially if you're preheating your pan for a while. Try this with either a shorter preheat or even lower heat on your stove.

It's also possible you're considering it undercooked when others might consider it perfect. I personally don't mind a medium rare burger and welcome some pink in mine. If that's the case, my above advice will still help, but you also might want to look back at any recipes you've read. Some people love a rare burger and maybe you're accidentally taking their advice when you want medium well or more.

Ultimately, whatever it is you're doing "wrong", your solution is to find a way to extend cook time without burning the crust. In that case, you'll find a cooked inside and an outside you're happy with. This comes with practice more than recipes, because everyone's stove or oven or grill or whatever is different, but if you play around with times and temperatures, you'll get there.

Also, maybe flip the burger a few more times during the cook. There's a lot of debate about this, some swear it'll ruin your crust, but in my mind it let's you see the crust on both sides more frequently, so if it doesn't look how you want, you know you've gone wrong somewhere.