I've never done a spin class, but I've done lots of other group classes.

What Zen said was right on. Only you know what you're capable of. If you don't want to stand, don't stand. If she turns the resistance back up, turn it back down. Don't let her bully or shame you.

They want you to come back, or else they wouldn't have a job. Make it an experience you want to go back to. Modify modify modify to fit your particular goals and needs. Don't let them tell you what you "should" be able to do. Only you know that. Be true to yourself.

Good luck, and do go for another try, with a different instructor.

Karen