How close is too close? (You can rant here)

Okay so before I start this I'd like to say I'm not being "petty" in this situation, I'm not trying to end their friendship or come between them, and I'm not trying to be "controlling" towards my boyfriend.
I've been with my boyfriend since January, and we're both in our 20's. Our relationship has been amazing since the start, except for one thing that keeps getting in the way, which is his best friend, who is a female. (I'll call her N)
So my boyfriend is an extremely friendly person.. he's one of those who just can't be mean to people and put his foot down. His bestfriend N (who is quite a bit older than me) is constantly trying to come between us, and in my opinion she's acting like she's a side chick.
She messages me every. single. day. about my boyfriend.. telling me that they've been bestfriends for 5 years, and how she "misses and loves him so much" (he's currently over seas), and we both have Snapchat and on there she's constantly bragging about the letters she sends to him and when he calls her. She posts pictures of said letters, which are VERY VERY lovely. She's also extremely rude to me. I've tried talking to him about this multiple times and he doesn't see anything wrong with it, he keeps telling me I have nothing to worry about that even though SHE DOES LIKE HIM, that I shouldn't care because he doesn't like her. I don't compete with other females for a guys attention, and I'm almost at my wits end. On top of all that, her sister likes him as well.


So here's a post where you can rant about relationships or share your relationship horror stories, hope none of you have experienced anything like this.
-Atka