Why can't I have regular cycles

Okay, I am really tired of having irregular cycles.  I came on Dec 3 and now it is almost the 3rd again and I’m just starting to have pms symptoms something I usually have a week before I start.  Now it seems that my period is going to come on a different time this month.  The past couple of times my periods have came on at different days.  This is getting so frustrating to me since I really would like to be able to calculate when I would ovulate but it’s hard cause I’m so irregular.  I wish my body would stop being so unpredictable and work with me instead of against me.