I feel LZ doesn't love A as A does for her, that's why LZ would not forgive him for this and that.Nobody is perfect. A seems pretty mature, if you leave him, you will regret a long time. I hope you can cherish what you have.
If you seriously had a relationship with X, it will be totally different feelings towards him now.Such a story happens a lot all the time. Just the one that is in it doesn't understand it.