how to stop overthinking

How can I stop overthinking my relationship, & constantly needing reassure meant that I am still wanted In his life. 
Here's was today's conversation -
Me : I just get scared you get tired of me & don't want me anymore or think you want someone better than me. 
Him : Sometime I just wanna be alone or with my guys you know you should be the same I never get tired of you. 
What's your opinion? Here I go overthinking the fact he didn't acknowledge the part where I said "think you want someone better."