Help me!!!!

Hi, This is the first time I've ever reached out really.. but thats not the problem! My problem lies in my ex girlfriend... Not that I don't still love her, because I do and she still loves me, so she says! The problem began when we split up about 3 months back, since then we've managed to retain a good relationship as friends and more, but I think I care far too much for her. Since our break up, she seems to be selling herself short and doing things she wouldn't have done before, I won't go into detail. But anyway, because of this, I seem to pry into her personal life that doesn't concern me which creates arguements as I won't let tiny details go! Just lately she's gotten all defensive about seeing mutual friends at the same time too! I know nothings going on because the guy in question is like her brother, but it's causing me to feel left out... Basically, I just want to know the best way to not care if you get me! We've had serious talks in which shes told me that we'd have been back together months ago if I laid off a little! I love this girl with everything I have, so why do I keep on ruining things with silly arguments and prying into information that doesn't concern me????? Thanks Steve