How do I stop overthinking?

I’ve (18f) been basically in a relationship for 6 months but a few nights ago he (19m) made it completely official. It wasn’t really something that was talked about a lot, I can’t explain it other than it was a relationship that had never had a title officially put on it. Anyways, I don’t know how to stop myself from overthinking when it comes to relationships. I’ve always had bad relationships, and before meeting him I took over a year to myself completely to just heal and work on me. Then I met him, and all those unwanted anxious feelings came back. Thoughts like “does he even want me” and “is he lying when he says he loves me”. I’ve never loved someone like this before. I feel a way with him that I genuinely can say is so different than I have ever experienced. I don’t want to ruin it with those stupid anxious feelings.