She hadn't been sure about anything since she'd been a child, but she was tired of allowing others to clean up her messes. The last time, she'd almost lost her sister. She wasn't risking a next time.
Sniffling again, Elsa straightened up, gaze firm and shoulders back, a halfway-convincing facsimile of the Queen she was born to be. "I am. You've done enough already. It's time that I solve my problems for myself." It was a lot easier to pull herself from her inner turmoil when she considered her responsibility to others, and this time was no exception. This time, it wasn't just her responsibility to Anna, or to her subjects, but also to the other residents of Marrowood. Maybe she couldn't contribute much to the effort to get everyone back home, but she definitely couldn't if she allowed herself to be crippled by her mistakes. It didn't matter how horrible they were - she owed it to everyone to do her best to get past it.
She'd likely carry this action with her for the rest of her life, but she couldn't allow it to completely wear her down. Loki's blatant concern just drove home her resolve. "Don't worry about me, please. I'll be okay."