Tough one. It's way easier to blame someone else and only focus on their faults.
I remember a while ago reading about "the Rule of Five". Jack Canfield (Chicken soup for the Soul) states that for every goal you want to achieve you need to take five actions a day to get you closer to that goal.
If that goal is improving a relationship it would translate to giving that person five compliments every time you criticize him/her...
The rule of Five is an excellent technique to start rewiring your mind in a sense - making you less critical of that person in other words. From a Gratitude angle Rhonda suggests penning down ten aspects of him/her you really appreciate. Who knows...it might just reignite that flame in your heart and make it burn even higher and brighter than before :)