A case of the green eyed monster?

Jealousy: Fearful or wary of being supplanted; apprehensive of losing affection or position.
In my opinion jealously cannot be defined by a simple sentence, said feeling remains an unexplainable emotion, one which leaves a person confused and aggravated at best. Not only that but it seems the green-eyed monster rears its ugly head at the most inconvenient and unexpected of times- when you think your satisfied with what you have in life and expect nothing more nor less or when one is in the process moving on from an ex they thought they had already escaped.
Now I’m not saying I’m not content with Gavin, because I am, I could almost be falling in love with him in all honesty. So why was I so rattled by the thought of M with another woman?.. And oh I was rattled, for the entire day my head was all over the place in fits of doubt and fury, if I am forbidden from speaking of my legitimate relationship what on earth makes him think I want to know where he stayed last night and with who? The audacity of that man to emotionally manipulate me, placing my own relationship in jeopardy to then- when the message we were not going to be reunited any time soon was finally drilled into that thick skull of his, just click his fingers and move on! The lies and the bloody cheek… See outrage right here! 
My concern lies with the nature of this sudden burst of emotion, is it merely a case of ‘I don’t want you so no body else can’- I’m not the most unselfish person in the world so it’s more than possible, maybe I just love the idea that it’s only me he wants and no other female can compare, my ego has been known to inflate to such a moronic size before! Or are there genuine feelings remaining, stuck in limbo, that I’m not consciously aware of? I should be happy to rid of the burden that’s caused me nothing but hassle over the past month, but instead I feel a little sad and empty? And on top of that I feel guilty for experiencing such emotions. I have an amazing boyfriend and I’m whittling over an insignificant ex? Explain the logic here because I’m finding it difficult to see sense at the moment!
‘Aggression is a form of affection’ I really hope such statement is not the truth as I have never wanted to hurt someone as much him today- well not for a while anyway. I’m pretty sure threats of ‘stapling his stupid head’ and  ‘severe stabbing’ were thrown around the office in the midst of insults and strops! Maybe the hurt I’m experiencing lies deep within my own insecurities, suddenly now I’m not good enough to want or need? Someone better has come along and my name is merely one that pops up on the list of unimportant people of ones past? Rationally I should be happy for him, he deserves to have someone and it was my choice for it to be with someone other than myself… It would appear I’m the one still holding on when in fact I was supposed to have let go first. Friendship was all I wanted and I may now have the opportunity to realise that goal, surely I should be encouraging this behaviour not resenting it?
Logically my head makes very little sense at the moment, I cannot force myself to feel what I know I am obliged to feel and I dislike such lack of control! Oh no