The returns haven‘t been bad.
The US outperformance was just crazy over the last decade.
Your investment lifetime is many decades, not just ten.
Stay the course and not chase performance over the short-term. That‘s recipe for buying high and selling low.
Also US valuations get more and more ridiculous. It cannot go on like this forever, there is a ceiling in valuations.
From a purely fundamental performance, US stocks just did slightly better.
Most of US outperformance over ex-US came from USD strength and valuation multiples increasing. Which both are unlikely to continue on forever.