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Some thoughts are like habits in that they they happen automatically because you have repeated them so many times. In CBT, you try to identify these automatic chains of thoughts, discover what triggers them, and then "unlearn" the habitual way of thinking. A little Googling led me to this app which has great reviews: https://play.google.com/store/apps/details?id=com.excelatlife.jealousy&hl=en
For example, you might make a mark in a notebook each time you have an urge to check her Facebook to look for clues about where she is to help yourself become more conscious of how that thought controls your life. Then, once you are good at noticing when you have that thought, you force yourself to think of a time when she was very sweet to you. Force yourself to do this each time, and slowly the urge to Facebook stalk should diminish. Another thing you might do as a part of CBT is ask yourself about the consequences of the thing you're afraid of. BRAIN: She's cheating on you. YOU: So? BRAIN: That would be the worst thing in the world! YOU: Why? BRAIN: Because that would hurt you. YOU: Why? BRAIN: Because it means she doesn't love you! YOU: What's wrong with that? I don't love every adult I've gotten to know. BRAIN: If she doesn't love you, it's because you are unlovable YOU: I have friends and family who care about me. Just because one specific person doesn't love me doesn't mean I am unlovable. Many people go through multiple serious relationships before they find their life long partner BRAIN: What if you don't find another girlfriend, ever? YOU: I look around the grocery store and see people wearing wedding rings who are old, young, tall, short, bald, hairy, and bald AND hairy. Many people find love, and it is statistically improbable that I will not, over the next 60 years, meet more women I connect with.
You get the idea. Retrain your brain and enjoy a less stressful relationship.