My ex-boyfriend and I are still talking (not seeing each other), because we both have feelings for each other. Although we were exclusively dating, I never called him my boyfriend, because I wanted him to change first. After 6 months of dating, he went on vacation with his family and started flirting with another girl. They were both posting pics on Instagram and flirting online. He told me she was his cousin for the whole month he was on vacation. He told me the truth when he came back, but he said he never did anything physical with her and he’s in love with me. According to him, he wanted me to see and get jealous. If that’s the case, I don’t understand why he would tell me she is his cousin, but whatever.
I can’t trust him anymore, and I specifically told him I can’t be with him and that we’re just friends. It’s been 2 months. I feel weak and ashamed that I don’t have the strength to let go of him. I know I need to get him to move on too. I kissed another guy 2 days ago to try to forget about him, which only worked temporarily.
If I tell him I kissed another guy, it would destroy him, he would hate me and never talk to me again. If I don’t tell him that, but we just stop talking, he would have a harder time letting go of me. Is it better to tell him the truth? I don’t know which one would hurt him less, and help him more. It is morally hard for me to lie, but the thought of him hating me is also hard to stomach.
LemonVibe 
First, never make conditions that the other person must change to receive something from you while in a relationship. You either accept them as they are or move on. This is a trap we have all fallen into, and it doesn’t end well for either party when it happens. The only exception to this would be if the relationship was well established and either one were in a destructive form of addiction. Otherwise it is a sure way of setting the relationship up for failure. Accepting each other for who you are is the key to unconditional love. It seems you are more in love with the ‘idea’ of who you want him to be more than who he really is. Both of you are trying to force the other to ‘be’ a certain way, going against the core self of who each of you really are. Wearing a mask to placate the other person is not healthy. A person can only maintain the mask for so long before the true self comes out and then all hell breaks loose. Being honest, authentic and genuine in nature is the only way to establish a true and solid foundation for a lasting relationship. You need to answer this question. Would you rather he wear a mask being someone he is not so you can have him right now, but in the end be more emotionally damaged from the lies of him wearing that mask? Honesty and clear communication are critical to ensure both are on the same page, understanding with clarity the condition of the other person. Your relationship right now is toxic for both of you unfortunately. You just need to decide which you want more, the mask or the real person.
It’s not your duty or problem to help him move on.
Don’t tell him about the kiss, because as your ex he’s not obliged to know your social life. That’s now all yours and private. Don’t stoop down to the level where you feel the need to start manipulating him with the “I kissed someone else.” I would also put him on the list on Facebook where he can’t see all your stuff and advise you to go ahead and unfollow him. The less you see of each other or hear from each other the easier it will be on the both of you.
Also, the weirdness with the girl. That’s just pure manipulation. Do you want someone to manipulate you in a relationship?