Nothing by mayhem

Is it possible to has feelings for two people at the same time? 
My boyfriend- when I’m with him, or speaking to him, or just not doing much else in life interesting he is the only guy in the entire world I want, he makes me smile, laugh, I miss him immensely when we’re not together and want to build a future around our relationship. Love is a strong word but I’ve been inclined to use it to describe the emotions I have towards him! But this sudden burst of anger and jealously I have directed at my ex is making me doubt the stability of what me and Gavin have.
How can his seeing an ex partner of his own affect me so much if there are not underlying feelings present? And if there are can what I think I feel for G be genuine? My head is in such a mess right now, I’m petrified of jeopardizing what I have fought for, what I lost M for in the first place due to doubt and uncertainty.
Decisiveness has never been a strong point of mine, it took me over two months to cut all ties with S, finish with M and settle with G- confusing right?! Am I just doubting the decision I made or do I just despise the thought of M being with someone else… Is my jealousy caused by a legitimate and logical reason (to which I am not entirely aware) or am I just scared of finalising the decisions I have made? Commitment has always been a tainted word in my vocabulary, with no one else to fight for my affection I am well and truly tied to that one person, it’s just me and G- together.
I feel as if I’m risking everything by daring to consider any true feeling’s directed at M, and realistically I don’t want to lose anything, I can’t imagine not having Gavin in my life. Maybe that’s where my true problem lies, I don’t want to accept I can’t have everything… Sacrifices must be made whilst reaching ones final destination. I thought I had reached mine with G, I’m sure this doubt and uncertainty will fade away and feelings of happiness and security will soon replace them, I just miss cuddle’s at the moment. I think missing his physical presence could the problem, either that of I’m subconsciously trying to ruin something that could potentially be great for me in an attempt to protect myself… It’s been done many times before!
Me and M are finished, maybe the hopes of a friendship surviving such a turbulent breakup was unrealistic, we can’t keep hanging on to something that is no longer there and I need to let go of something I no longer want.