In my head so much!

I am in a new relationship of about 3 months. I went through a divorce about a year ago and am new to dating. I am in my mid 50s and have my youngest daughter ( a senior in HS ) quarantining with me.
My concern is the thoughts that I constantly have in my head where I tell myself that my partner is losing interest and things will not work out.
I am trying to apply the model of changing my thoughts and then the feelings that follow but am struggling to apply this to my situation,
The result of the negative feelings is feeling insecure and down. I think that Iā€™m overthinking things and need to really change my perspective in order to help have a different result as per the model.
Any advice here?