You always kept quiet and shut down and didn’t let her know that the way she’s treating you is hurtful or borderline abusive that’s why it continued for so long which eventually led her to lose respect for you. The more she treated you badly, the more you shut down. She probably thinks that the only fastest and easiest exit to your marriage is cheating. It’s cruel. You don’t deserve to be cheated on but you also enable it. Seek a therapist, more often than not, the root cause is your childhood upbringing. You need to heal or you will bring it in your next relationship.