bro, there are a lot of factors that come into play when dealing with this.. she also might be so insecure about how YOU handle your relationship with her.. or she might not be satisfied with the kind of love and care you're giving.. so she ends up having to compare you and her ex's cz she's probably evaluating you and that she has this set of EXPECTATIONS on how the relationship should work between the two of you..
but still, it's ANNOYING.
bro, remember this.. one of the MOST HURTFUL thing that a girl can tell to a guy is that he's a LOUSY lover especially when compared to her ex's..
believe me.. i've been there, done that.. like what @Ms. Beau said, you have to command and DEMAND respect from that girl..
else, the BEST solution is to LEAVE..