Help overcoming jealousy

My boyfriend used to like one of my friends before. She didn't return his feelings but they're still close friends. I was okay with it and didn't give it much thought until I was talking to my bff about it an she asked "don't you ever worry that he's only with you because [other friend] rejected him?" I said no because we'd already talked about it and that was all we said on the subject, but she's planted the seed of doubt in me and now every time I see them interact, having playful banter, etc. I can't help but feel insecure. I had never felt unsettled about this subject until that friend brought it up. Nothing else has changed in our relationship, and I haven't said anything to him because I know I'm being irrational and jealous, but I feel so uncomfortable! I know he loves me, I can feel it in my gut, but I can't shake these fears.