Most Helpful Guy
You're 18 and this guy is 26 with a baby already, another en route but also cheated on his then girlfriend with you. Point being, is this issue really that difficult for you to comprehend? This is not a difficult decision as you cease contact with everyone in that entire situation, steer clear of each and every one of them and move on with your life while hopefully meeting someone without that much baggage.
I am fairly sure you won't heed any advice on here anyhow and will simply continue to go after this guy but consider this: If he has two kids and a girlfriend and abandons them to have sex with you, what do you think he's going to do with you, a person who he has not a single shred of connection (baggage wise) with? In other words, he will drop you as soon as the next best thing comes up so my advice stands: move on... quickly.
Most Helpful Girl
"I don't have kids of my own I'm 18 but I'm very mature. ".
You are not mature.
Mature people know how to put their selfish needs aside, look at situations for what they are,
and make decisions that will not negatively affect them/others.
You walked into a situation knowing the damage and destruction that
could possibly occur from this.
Just like she treated him good, and he left her.
In time, he will do that to you.
Do not be a fool!
Your poor choices will come back to haunt you.
"What can I do? To prevent him leaving me?"
I'm sure the mother of his child asked herself to the same questions at one point.
If a man wants to tray he will tray.
Nothing you do or say will prevent that.
Walk away from the situation while you can (because you have no attachments to him).
Enter into a relationship with someone who is single and not involved with any one else.