Being Loyal Isn't That Difficult
Take the honesty out of a relationship and what you are left with is a lot of caring and a big question mark as far as trust is concerned. And nobody wants a giant question mark sitting in the middle of a good relationship tanking up the valuable space where trust should occupy.
Trusting another person comes down to a personal decision and if that person decides to let you down...Remember they made a choice to do so and since everyone deserves to be with a truthful and caring person if you find they can no longer be that, then why stay any longer?
It's very hard to trust a person to do the right thing after they have crossed that line and staying in the relationship may cause you unnecessary strain in your life. Let them make the choice because its ultimately up to you how many lies you are willing to put up with in the end.
Never apologize for leaving a situation to make yourself happier. your health matters and your happiness is important and if the person you are with cannot understand that well then they can be dismissed. Never settle for less than you deserve when it comes to ANYTHING! Life is too short to even begin to live like that.