I'm a woman, and I have been with my husband for 18 years. He is 22 years older than me. When we met, I was still recovering from a nasty divorce where my ex repeatedly cheated on me. After much thought, I rejected monogamy. My (now) husband was fine with having an open relationship. For the first 10 years, we had fun as swingers.
If you are in an unhealthy marriage that has been dysfunctional for years, it may be hard to accept the reality that your marriage and/or your spouse is not going to change. You've invested time and energy into this relationship and don't want to give up now. Maybe you keep thinking that once life settles down, or you get that new job, or the kids are more independent, then things will get better.