I CAME across a thread on Twitter recently. The tweets were from a girl who fell in love with a guy who's already taken. She didn't know that he was already with someone else and the guy purposely lied to her about being single. The guy was actually very sweet to her and made her feel so special. They went to several dates and shared a lot of romantic moments together. Until the guy's girlfriend sent her a message asking her about her relationship with the guy. That's when she discovered that she was fooled by the man she have learned to love. Fortunately, she was smart enough to let the guy go and decided not to get in between their relationship. She explained her side to the girl and even forgave the guy who lied to her (which is something that's so hard to do considering her situation).
You should remember that not anyone who exerts effort to make you feel cherished is always loyal and honest. That person might just be having fun or is simply making you a second option. Which is unfair. Sadly, some people are really like that. There are cheaters and there will be victims. But if you learn that he's already taken, then don't chase. Let go no matter how much you love that person. You deserve better. You are not a second option. If he tries to talk to you using his sweet words, don't give in. The fact that he cheated on his partner to be with you remains intact. That alone is a loud warning sign for you to stay away from that person. If he was able to cheat on his partner, he can absolutely do the same thing to you.
Don't stay on that kind of relationship if you know that you hurt someone else for you to be together. You cannot be fully happy if someone's hurting because of you. And always remember about karma. It will come to you and it might be worse than the mistake that you've committed. Being in love is not an excuse to hurt other's feelings. Think about the person who got cheated on. Think about her feelings and how much your relationship hurt her. Even if he says he loves you, you can't seriously buy that. If he truly loves you, he shouldn't have lied to you in the first place, right? He made you believe that he's single. He showered you with a bunch of lies. Most of all, he made you a mistress.
People can get stupid when they're in love that's why you have to be very careful. When you're in this kind of situation, take a deep breath and remind yourself to do the right thing. You know what's right and what's wrong, you just need to have the courage to follow the right path no matter how painful it can be. You cannot snatch someone else's partner just because you're also in love with him. You have to step aside and let them deal with their mess. It's okay to get hurt and angry but you have to cut ties with them. Remove them from your life and let them fix their relationship (if they still can). Continue living your life and learn from that experience.