Is a choice! Even when things get EXTREMELY hard you can choose to love your spouse. My husband and I have been through a TON in the 2 years we have been married.
You must, however, realize it is not letting the other person walk all over you! It is about not stooping to the level of the other person when acting stupid. As humans we ALL make mistakes, but that cannot be our excuse for making them. We must apologize for what we do and not expect other person to forgive, but you have to know they will never FORGET! Dint expect them to.
Earning trust back is another hard thing to do, but not being able to trust the other person is even harder. It is easy to get mad at the other person for not trusting you but remember that you are truly mad at yourself! Remember be tough on yourself and tender to your spouse.
You also shouldn't hold what your spouse does over their heads, it will cause both of you to resent each other.
I STRONGLY suggest doing the Love Dare to your spouse even if things are going well, but especially if you are finding it hard to do anything for or with them. It really helps you look at your relationship from a different view. The Love Dare is from the hit movie Fireproof, you Can buy the book at target or walmart or you local bookstore. You don't have to be a Christian to do it, it is the concept of all of the things in the book, they apply to all marriages. It is about true love! the purest form of love!
there however are things a spouse can do where even biblicaly it is OK to divorce such as adultery. but that also doesn't mean you cant make it work. but it is between the two of you no one else can tell you what to do.
Also you can't fall out of love! since you never fell into it to begin with. yes lust you can fall out of and that's when REAL love comes in! If love was easy we wouldn't have the divorce rate we have, you need to work at it.
Don't be another statistic when you don't have to be. You choose to love.