I really need advice on how to handle this. My FP at the moment is a really cool person, she sees me as her bff but I see her as, well, my FP. We have different ways of expressing feelings and though she says she loves me all the time and even told me I'm her #1 friend, I can't help but feel jealous of some other people she hangs out with and get to see her more than I do (due to our age gap, different schools etc.). Not to mention she has many other talented friends too, some of which even have a crush on her and and I feel like I'm going insane. 

I'm obsessed not only with her, but the people she's around, what kind of person they are, why is she friends with them,... and I always feel like I have to be better than them so she wouldn't replace me with them. For the past 2 years or so, she's been hanging out with this other girl and they're gradually getting closer and I'm just helpless because I'm busy with work and isn't the type of person who could hang out with her like that (they play games together, study together, both are in the same circle of friends at my FP's school). 

Though it is not the first time I've had these thoughts, it is the first time I've contemplated on my thinking. I think about killing the girl my FP's hanging out with. Not just stabbing her or any gory stuff for relief, I actually have a murder plan in my mind. How we could be alone together, what kind of weapon would I use, how to put on a disguise so I wouldn't be suspected, how to manipulate witnesses, how to create the perfect alibi, how to set it up so people would think she was killed by a stranger. I KNOW I wouldn't do it, but it's getting more and more intense every time I switch FPs, and I'm so terrified of myself because I don't feel anything while thinking of those things. Just calmness, and relief. And happiness.

Is it considered a coping mechanism? If so, is it a healthy one? How can I resolve my jealous tendencies without distancing myself from the friendship and/or my FP? Is there any way to guarantee she wouldn't replace me like she'd assured?

Therapy is out of the question right now due to multiple reasons. I'm exhausted of lashing out over the slightest thing, of overthinking when my FP go hang out with others.