Why is she still in my head so much l

It was her birthday yesterday I didn’t text her she tried to Hoover the other week I didn’t bite but now she’s just on my mind constantly even when I’m with my new gf and it’s not fair to her she’s so much nicer and healthier for me than my ex but for some reason I have this desire to go back to her but I feel like that’s not what I really want but at the same time it is I feel like all the progress I made from getting over her has just receded and im confused and frustrated