I think you and your suitemate’s biggest problem is lack of effective, friendly communication.
Another problem is a misunderstanding of motive. Here is one thing you said.
You jump to the conclusion that your suitemate is making the decision to get in the shower at that time for the sole reason of making your life unpleasant. That would be exhausting for her to keep up in this and other scenarios. Instead of assuming the worst in this particular situation, consider this.
Maybe your suitemate wakes up, lies around in bed, and procrastinates away the morning watching videos or reading. She hears you return to your room and she realizes that if she doesn’t get in the shower right way, she’ll have to wait 20 or 30 minutes until you finish your shower before she takes her shower and heads out for classes. So, she rushes into the bathroom to take the shower she should have taken 30 minutes earlier. If you don’t assume she has nefarious motives, it might help you feel more comfortable about your suitemate.