Hey MrRich,
Can't shed light on your personal situation but I do empathize as I feel like I am going through something similar, and I have three theories. Just a bit of a preface, we were together 11 months (I broke it off but kinda didn't have a choice coz he was treatin me like shit) and broke up 6 weeks ago. First 4-5 weeks I actually felt fine and thought it was all going to be smooth sailing but this past week I have been thinking about him NON-STOP and it is really confusing me.
My first theory is because I have returned to Uni and things are really full on and I have been really tired and I think when we are feeling that way nostalgia can set in (same as when we feel angry or lonely or even hungry). So has anything changed in your life of late that may make you feel a bit 'lower' than usual?
My second theory is I feel like I am over him and therefore ready to date again but totally don't know how/when/if that is ever going to happen and feeling a little bit overwhelmed and lonely and would love probably just love some sort of attention. Sounds egotistical but just being honest.
Third one is he is my 'default' guy.. the last person to have my heart so my feelings automatically aim for him. I am convinced if/when someone else comes on the scene, no matter what capacity, he will exit my life for good.
Push past it mate, it is a temporary madness.
Some people are drains and some are radiators... Keep clear of the drains and hug the radiators!