Ego Problems caused by PMO?

What you are describing is called Puberty. It causes 15 year olds to be difficult to get along with - which is fair because it's a 15 year old's job to begin to feel independent. Attempting to grow up while the family isn't providing support can cause depression. Depression leads to wanting to stay in bed all day and ignore the world.

Children don't mind a lack of privacy because they've been dependent on adults to provide all their care and education.

Teenagers want privacy and there is always a problem with a teenager, a human starting to feel like an adult, and parents who want to keep their child safe. Parents also get frustrated because they expect a teenager to act like an adult some of the time and they want the teenager to understand they are not an actual adult the rest of the time.

From your point of view, you caught her in the act of doing something a child shouldn't do. From her point of view, you committed a violation of her privacy.

To start the process of her forgiving you for the invasion of her private space, you need to apologize to her. Yes, it's your house and you have the right to go where you want when you want to go. You cannot, however, expect your stepdaughter to start acting like an adult part of the family if you don't give her some space.

Do not give her the "nonapology apology." By that I mean something like this, "I'm sorry if you were offended by my actions." That isn't an apology, it's an insult. An apology would be, "I should have knocked and I'm sorry I didn't. I'll knock from now on the same way I expect you to knock when you want to come into my bedroom." There is a door to her room and to respect each other, adults knock before entering and wait to be invited in. Shy of believing she's in danger, start treating her the way you'd want to be treated and knock before you enter.

The problem is the words "act like." Parents want their teenagers to "act like" an adult. But, parents want to make sure teenagers know they are still children. Is there any wonder she is withdrawn and has a fragile ego?

She's only 15 and puberty lasts until 21 in most people. Do you want 6 more years of her hiding away from you or do you want to foster a relationship based on respect? Hormones in teenagers give them the same mood swings that menopause in women does. Yet, parents tend to dismiss hormonal swings in their children while respecting those same hormonal swings in their mothers.

Both you and your stepdaughter have some growing up to do.