I am friends with a guy named Kevin and at the beginning of the school year we dated for like 5 monthes then broke up because we used to be like brother and sister and we thought it was better. Well after like 5 days we decided to try again and we dated for about 3 monthes and one night I was texting him and he said he had to tell me something and I was like what and he said his friends stayed the night I was like so and then he said one was girl and his other guy friend mike (which I knew) I was like well did you guys do anything? And he said no of course not. I didn't like that and I didn't trust him so I asked mike and he said he wasn't even there so I broke up with him and kevin and I talked about getting back together but I don't know he needs to grow up first but how do I get him to know I'm serious?

Get him to know you're serious about what? Why did you break up with him this time?
Why do you keep messing with the friendship by dating? You keep breaking up, leave it alone.