I can't believe I believed, everything we had would last
So young and naive for me to think, she was from your past
She thought that the other woman was just an ex-girlfriend and would stay that, an ex. She attributes her foolishness of thinking he would stay with her and be good to her to her young age and relative naivety in relationships.
To help improve the meaning of these lyrics, visit "Me, Myself & I (Remix)" by Beyoncé (Ft. Ghostface Killah) Lyrics and leave a comment on the lyrics box