OOOOH I have to elaborate on Quath's thought.
In any given relationship-not just between lovers,
NO ONE can make or break the relationship but the people in it.
This is something Midnightsun and I were discussing recently.
My stepson's mother has spent 12 years in emotional turmoil over me 'replacing her'. It's not possible. I LOVE my stepson as my own-and he loves me as his own. He calls me mom. I sign my letters to him MomLR (ok I use my real initial, but you get the picture). She LOATHES that he calls me mom because she feels like it takes something away from her, but the truth is-it doesn't.
My daughter (the oldest) calls her auntie mom and she calls me mom (sometimes mommy still) and she calls her stepmom mom.
She calls her father dad, she calls Maca dad and she has two other guys she calls dad as well!
It's a term of endearment, but it doesn't replace anything.
Maca can't make or break things for GG right now in our relationship (GG and I are struggling through some learning steps) and when Maca and I were struggling GG couldn't make or break things for us.
Your SO's other girlfriend can't be you, can't replace you, can't make him love you less or more, can't make you more or less sexy to him or more or less good in bed either.
ONLY YOU can do those things.
BUT you CAN improve ALL sorts of things about yourself by choosing to and you can improve all sorts of things about your relationship by improving yourself and getting to know him EVEN BETTER too!!
"Love As Thou Wilt"