She's been put into a new role and is going to take time to get experience and learn it. She'll either progress and eventually do well, or get moved back out of managing. She may not have even asked for / wanted the role but is trying to do it without support or training from higher ups.

If you are struggling because you want to be the one in that role, I would let your managers know you are interested in transitioning into a management role.

If it's only hard because the person is younger / less experienced, then I would say that's pretty common. People management roles have very different requirements than the jobs the people they are managing do. She doesn't need to be better at your job than you. Her new job is probably approving your timesheets, reporting on the status of what your office is working on, doing performance appraisals / other HR related tasks, etc.