Why am I worrying so much

S the guy I've been in love with asked me to be his girlfriend on the 1st of February we were kinda together wince 2014 but January we were split up and he was seeing a girl and he didn't end things with her until 4 days after we became official and the day after I moved in. He told me he was seeing someone but didn't want to pursue anything further because she wasn't me. He said the deal was that they stay friends and he has talked to her so so. But I saw her messaged him on face book saying something like " if your looking more as a parting deal I would suggest some kind of food place" why am I worried about all this. Oh he's 26 I'm almost 20