I think the engagement thing was a huge mistake, it probably scared the shit out of her.
If you're not ready for marriage for three years, don't get engaged for another two. Engagements are for when you're ready to start planning an actual wedding.
I would go to her, tell her everything you're feeling and tell her maybe you guys should dial things back a bit (forget about the engagement thing for awhile). Tell her you'd be hapier just being with her. Maybe she'll relax then.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi