What do I do about this?

Im 15,and my boyfriend of two years ,who's is 18 makes me upset and worried at the same time, we will be texting for a while, and then he disappears for like three days, and then tells me he was busy, ........well I get a little worried because I think that he is with other girls or something, and then I get upset because he should at least tell me he was going to be busy.. .......what do I do,