At 18, in love, and newly married, you should be having a wonderful, wonderful time. Not this. You may be ready to be married but he has been showing you again and again that he isn’t. I’m so sorry that things are working out this way for you. When you’re not upset and angry, you must be very, very disappointed and sad.
Unfortunately, the two of you have been setting the terms for how you’re going to be married ever since your wedding day and it’s not pretty. He cheats and lies. You worry, get upset, and look for evidence. He gets caught but acts like he’s sorry. You forgive. And he’s good to go again. Your trust is further eroded. He thinks he’s figured out how to be married and single too. If you don’t do something really, really major, this is what your life will be like forever. I think you deserve better.
It’s doubtful he’ll believe any ultimatum on your part at this point. As upsetting as this is, I really think you need to see a lawyer. Find out what your rights are and what would be involved for getting a divorce. Then ask your husband if he really wants a marriage or would be just as happy to have a divorce. If he says he wants to try (and you still have it in you to try too), insist on couples counseling immediately and hold him to it. Within a few months, you should know whether or not a marriage with this guy will work. If it doesn’t, you will know that you’ve done everything you could. Then, please, move on. Better to have a short early marriage and learn from the mistake than to sign on for a lifetime of doubts and anxiety and betrayals. You deserve someone who will love you and cherish you and be a real partner to you.
I wish you well.