Shell, it's been 5 weeks.. give yourself a break. you need time to get over this, and you will. experience tells me that when you're constantly thinking of someone when they're not thinking of you, it's obsessive behaviour and detrimental to your health.
stop having expectations of yourself. just go with the flow but accept that's it's over. he has moved on (even thought it's strange and hurtful circumstances), and he sounds like a nasty piece of work. in time, you will see all this for what it is but in the meantime ease up on yourself. accept invitations to go out but just for the purpose of going out, not to try to find a replacement man.
chin up, you're getting through the worst of it.