Rhythm game difficulty is fine at a level that suits you

In online co-op, you don't have to force yourself to match others. If it's difficult, just choose Easy or Normal, whatever is comfortable for you, and give it a shot. Just reducing mistakes alone will earn you points, and when you feel more comfortable, you can try Hard or Expert. The key is to have fun with everything. If you keep at it, you'll naturally get better. (Individual differences apply) Keep building it up steadily with your favorite character.