I struggle with this same problem... One thing that I've noticed is that when I journal my feelings out and get them on paper, my mind tends to get clearer and clearer. I get so worried that I may forget what I'm thinking, so my mind puts these thoughts on replay.
Just remember to breathe and take things easy. Make a list of reasons why you are blessed to be with this person, and when these thoughts come back, read them to yourself and maybe that'll help too