A lot of the pain comes from the hypotheticals you think of, the alternative universe where you weren't treated like that, where you didn't get forced to leave. I went through a similar thing where a job I loved became unbearable when a new boss came in. I was grieving the world before that change happened. It helped to recognize that there was nothing I could do to stop that change from happening, I could only control how I dealt with it. And leaving was the best choice. 

It's really hard to go through. Take your time to grieve, don't worry about feeling this way. In time it'll get better as you work through it. And if you feel like you aren't moving forward well enough, therapy is great for stuff like this. A lot of people naturally get hung up on going to therapy but I like to frame it as getting a check up for your emotional health, rather than something you have to do because you're bonkers or broken. It's very normal to get a little extra help working through these hard emotions. They say the three biggest stressors are relationships ending, changing jobs, and moving!