Trust issues am i right or wrong? Please help

Hi im going through a tough time at the minute , my partner has told some silly lies at the begining of our relationship she said she hadnt been in contact with her ex partner , i found out 6 months later that she had been in contact with him by phone calls and texts , i dont know what they were about but she has reassured me that it was nothing , i really want to believe her and trust her but she has broke our trust and me coming from a relationship before her which i was cheated on makes its so hard for me to believe what she is telling me , i really do love her more than anything and would do anything for her , i just have this part of me that finds it so hard to believe that she is telling me the truth , we talked about a polygraph test last night which made her angry and sad , part of me thinks its because she is lying and part of me things it is because it is an insult to her , she gives me no reason now days to believe she is cheating on me … its just the past i am so focused on and cant seem to get past it , it really is starting to affect my mental health ( anxiety , depression) , please help i love her so much