Sharing Our Innermost Thoughts

Hii!! Lol Idk what to say!!

Sometimes when I get in a little argument with my best friend I feel a burst inside me and then I avoid the argument by just agreeing with her. 
We have been best friends for almost 6 yrs ig and I feel like i don't tell her everything not as in things but my feelings, the things I feel about her or anything. But I feel it's not fair for her as she tells me almost everything, I accept there must be some things too that she doesn't tell me, but I am sure it won't be as big things as as feeling or thoughts.

Idk what I should do as I keep feeling that there will be a big argument betn us soon and I am scared it will be too big and affect our friendship…