You have to try and make other friends yourself, so yr not so reliant on yr best friend. You've been friends a long time&I'm sure she realizes yr value& prefers you to the other girl, even though it doesn't seem that way to you.
Don't show the other girl you're jealous as that will make you feel and look bad. Stay away when the other girl's there and let them have time alone while you go out and do something else- if it makes you feel bad to see them laughing together.
The more grown up you act about this the more yr best friend will appreciate you& value yr friendship. The other girl might just be a passing friend.
Occupying yourself with hobbies& doing stuff you like on yr own or with other friends will help you feel better. You should try to know this other girl more one on one. If yr best friend likes her, she must have some good qualities. Don't let jealousy get the better of you.
Write a journal of how you feel and tell yr mom if yr close. Also tell yr best friend that sometimes you feel sad and jealous when she's always with the other girl- and that you'd like more quality time with her too.