How should I roll with someone much better than me?

Rolling with higher belts is one of the best learning opportunities in BJJ if you approach it correctly. Set small goals like surviving for two minutes from a bad position or executing one specific technique. The higher belt will be working on their game as well, often giving you opportunities to try things. If you flail wildly, they will shut you down immediately and you learn nothing. If you move methodically and try specific techniques, they often let you work and may even coach you during the roll. After the round, ask what they noticed about your game. Where are you leaving openings? What should you be doing differently? Most upper belts enjoy helping lower belts who show humility and willingness to learn. Do not be discouraged by getting submitted repeatedly. The higher belt has years of experience, and the fact that they can submit you easily does not mean you are bad. It means they are good. Focus on the small victories and use each round as a research session for your own improvement.