Paranoia killing me. :( Help?

Like you said you have low self esteem - you have to stand up for yourself in this situation. If you like the guy and you see him and your friend flirting do something don't just stand there and watch. Just because they have an attraction doesn't make you invisible. Great your bf and your best friend get along - we all like that! Maybe talk to your bestie about it - that you feel this way. She can offer you a shoulder and at the same time can clear up any lingering doubts that she may be attracted to your man.