My ex is living with someone else

Hi all,
I just wanted to ask a question and I am struggling to process this in my own head. my ex and I broke up three weeks ago and she is already living with someone else and spending a lot of time together. Less than a week after we broke up they also went for an all-day hike. I really miss her and obviously want her back but also not sure how someone can possibly move on so quickly.
We had been in a relationship for almost 10 months.
I acknowledge that there’s a lot of things I need to do to become a better boyfriend and I am seeing a psychologist to help me work through that.
Is there any possibly way to “win” her back even after this?
Thanks.