How do you deal with an ex moving on and falling in love with someone else immediately?

I was broken up with recently, after having been with this woman for 6 years. She felt like things werent working out despite having sat down and having a discussion a week prior to the breakup on what we needed to fix. Anyways, shortly after we broke up I was going to get my dog from her and learned she was with a coworker she had used to work with, and was talking to a lot before we broke up. She told me she felt a spark with him and immediately fell in love.

I know I should not be worried about it in the slightest, but its honestly really hard to let go of a relationship that lasted so long, and even harder to watch her move on to the next like it was nothing for her.

I really dont know anymore lol, all I want to do is get this out of my head and move on with my life but it feel impossibly hard.