I'm thinking about cutting off my best friend. Why? Not even I understand my own logic here, but here we go:

My best friend has been talking to a new woman. I'm a straight man that has never felt any romantic affection towards him, yet I'm irrationally jealous. I'm feeling so disheartened by it that it seems like a waste of time to try maintaining the friendship. I never had this problem with his ex or any long standing female friends. To be clear, my friend has done nothing to indicate that our friendship is changing as a result of this. Its 100% on my end.

So what the fuck? How can I know I'm being irrational yet still feel so strongly about this? Just earlier today he thanked me for always being here for him, I should be feeling rather secure in our friendship. 

I haven't expressed any of this to him because I realize its insanity on my part, but the mere mention of the topic throws me off balance all day and gets me in a very negative mindset.

I'm marking this as venting, but any advice would be appreciated.