Please help me stop being so paranoid and jealous.

I trust my husband, but I am irrationally paranoid. He started working closer with another woman probably about a month ago. Basically, she gives him his assignments, but they don't work side by side. They hang out together on breaks. 

When he talks about work it's always girls name this and her that. I've brought it up how it upsets me sometimes and he's always quick to reassure me. 

She's in a happy relationship with someone else and my husband says she wants to officially meet me. He says they both talk about their families...

Some days I am okay with their relationship. Days like today I'm not.

I keep feeling like he'll leave me for her. Or if something happened that she'd be the one he'd turn to not me.

He's given me no reason to doubt him, I just feel awful. 

I've always been like this with certain female co workers. I know part of it has to do with how unhappy I am with my weight and how I compare myself to everyone.

Doesn't help that once I was right and one of his coworkers was trying to play wife with him at work, even though he continuously pushed her away. Different job. Different person. We used to work together. 

Anyway. Suggestions?