I think Mani may be right, you could be a bit lonely.
I have certainly gone through it, thinking about a past relationship remembering all the good times and being nostalgic when in fact there were also 101 good reasons why we were not still together.
The sun is shining the evenings are getting lighter, make plans, put them on a calendar and make sure you have at least one entry every couple of weeks, something to look forward to. You will soon stop thinking about him in that way and who knows you may soon have someone else to think about?
Enjoy yourself and get out there, lifes too short..........
I want 3 things on that calendar in the next half hour - go!