Don't make a fool of yourself being faithful to someone who isn't. Don't believe their lies & excuses. Believe their actions but most importantly, believe their habits. Be faithful to someone who deserves it.
Sometimes you just have to accept that some people are shitty humans and stop trying to see the good that isn't there. - It's takes a lot for me to believe someone is a shitty human being, but once I do, I'm done.