The triggering 3's. 3 days, 3 weeks, 3 months. For some odd reason when we approach the 3's our feelings are triggered or heightened. Just like the firsts, first holiday, first Bday, first anniversary without them. as mentioned its grief and the only way through it is to allow ourselves to feel it rather then invite them back so we don't have to feel it.
It is good you are going to therapy, know that therapy takes time it's not instant. Maybe think about finding an al-alon group in your area.
When your thinking gets stuck on who he may be with, try and replace jealousy with empathy and be thankful you are not whom ever he may be with today because YOU deserve so much more.
Try and do that self talk from this being some kind of a lose to "you are winning!".