It's a pig when you feel in love but that doesn't make you feel good about yourself. The mind has a way of playing such tricks when we are feeling insecure.
Can you truly say that this person is thinking of your needs and if not then why don't you think that you deserve someone who is able to give you the consideration you have tried to show?
Who knows why people cheat. In an open relationship there are not the usual risks to being honest, loss, endings. So maybe she has something going on that is about the thrill of a hidden liaison. And if that's the case how can she maintain that thrill without repeated deceit? How can you accept that and would you want to?
Maybe sit and think about what you want from a relationship and try to be objective about that outside of all other considerations. Be selfish for a moment. Tell yourself you deserve to be happy and be treated with respect and consideration. See what you learn and then compare to what you have.
(as to your mate... hmmm. You maybe have low expectations of friends and partners. Why is that?)