Why do I feel mixed emotions and sadness about my ex’s new job and big life change?

So my ex and I have been working together for about 7 years and he's always talked about leaving and I've been prepared for him to leave. We haven't had the greatest relationship, that's why im his ex. He talked about getting another interview with this corporation that he failed to get into before. We decided to leave early and head over to his place. His mom was over there taking care of the dogs. While I was there I asked his mom about her recent trip. She's moving to another state next year. Then my ex got a call.
It was the people telling him he got the job and his offer letter was sent to his email. I was sitting next to him as the recruiter talked and his mom was nearby and she started crying. I ran to hug her. Both of us were happy because he wanted this job so bad. The job starts next month. I feel happy but a little teary eyed too. I think it's because everyone's changing and I'm used to routine. Next month my routine is gonna change and I can truly start healing. Whether or not im ready, it's going be cold turkey. I think im just nervous
Why do I feel mixed emotions and sadness about my ex’s new job and big life change?
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