How can I stop thinking that he doesn't love me?

If my boyfriend doesn't respond to me, I start to get very nervous and think that he has fallen out of love with me and plans to leave me. Today he stopped answering me a few hours ago (I didn't write anything important). I cried for an hour and now I can't sleep, I'm nauseous and I know that in the morning I will feel bad. This isn't the first time. I don't know how to stop