Do you constantly think about the past and don't know what to do?

I broke up with my boyfriend three months ago. He left me and is now dating his ex. We met with him for about 2 years. And it seemed to me that everything we have with him is real and forever. But lately we have been constantly arguing, cursing over trifles. It seemed to me that he became inattentive and irritable because of any little thing. In the end, I told him everything. Everyone talked a lot. And so we parted. I felt in my heart that something was wrong a month before it all happened. And it seems like at times I stop thinking about him, and in the evening my mood drops and I just can’t switch. A couple of days ago I even thought to call, I found an excuse. But she took matters into her own hands. Tell me how to turn this page of life?