Should I feel so on edge?

My boyfriend and I have been together about 7-8 months now. We spend everyday together so I don't know why I feel so paranoid and unable to completely trust him. He is good friends with a girl he used to be sexually involved with. They were friends for a couple of years before they became intimate, and she cut off the sexual relationship a couple of years ago. Her and I don't happen to get along, although we attempt to be friendly to each other. Last night we went out with his friends, I began to feel paranoid when I saw he talking to her so I went for a walk. I became anxious when I was alone and wondered if he was happier being around her, so I impulsively sent him a message telling him I wasnt feeling well and going home, to which he told me ok but he wishes I'd come back. I thought maybe he'd attempt to find me but when I returned I sat nearby watching him talk to her closely for about 10-15 minutes, and he looked so happy with her, without a worry of where I might be. It gets to me because I keep feeling like him going to her once I'd gone means he's replacing me with her. How can I get over this fear and just be happy with him? He always tells me I'm his life since we met and that there's no other girl he'd rather be with, so it must be my own subconscious?

Stick to your intuition, it's almost always right.