Every time he leaves me for her, my heart dies a little

Dear Best Friend,
So I posted the subject line above anonymously and people implied that I was the other woman. Am I?  My female intuition tells me that you're back with your ex or involved elsewhere. Thus every time we have this break between us, I get lonely and very sad that I don't know what to do with myself at times. I lose my motivation and spirit. It is toxic just being in such state, in connection to you. Someone posted it that there was no agreement of being exclusive thus, such thought of the heart implied that we are anything other than friends is invalid.
I lose my lights at times and you are my light at the end of the tunnel. I need you to find my ways and I need to walk through the darkness alone to get to you but YOU, my dearest cutie pie, don't know that. That I am in love with you.
My light, yours - nothing,
Tan