Why do I feel this way ? I (33F) an in a relationship (32M) and feel lovesick

Hi, I really need to understand the intense things I'm feeling, and above all, stop the suffering. I am in a 6 months relationship with my boyfriend, everything is fine, and somehow I got intense fear that we could leave me and fall in love//be subjugated by someone else.

As he and I are colleagues, this fear attaches itself to another female colleague in particular ; for she's cute and might attract him, talk a lot and make jokes. We share the same office room with a fourth colleague ; also, no one knows at work that we're together.

I feel sick and often cry about that, for all the pressure I put on myself, the scenarios, the way my heart ache when she tries to make contact and silly jokes with him. She has a boyfriend but the rare times she spoke about him were in quite negative terms.

Any advice or person that felt this way too ? What might help (I just booked an appointment with therapist that will take place soon).

Thanks