HELP ME!

so me and this boy have been having a thing for about a year now and we are really into each other but we want to go out next year when summer is over. recently i found out that my best friend likes him and that she wrote him a love letter and sent it to him. she failed to tell me this info until my other friend did. i confronted her and she lied to me later on that day telling me the truth. she flirts with him in front of me and knows how much it upsets me. i was speaking to the boy and he said she had done nothing wrong taking her side in the argument. i made up with her because i really didn’t care anymore but she came up to me saying oh is it ok if i go on a picnic with my best friends who are her another girl, the other girls boyfriend and my man. i said yeah sure but i have kind of had enough of everything what do i do? x