Girlfriend thinks I'm constantly eyeing up other people



  I'm 21 and have recently been diagnosed with ADHD, looking back now I have always showed signs of it since a young age ( easily distracted, bored easily, daydream, completing tasks on time, really forgetful etc.). My girlfriend has got very annoyed with me on multiple occasions because when we walk along the street I tend to drift off and would keep on looking at things around me, whilst not actually paying attention to what I'm seeing and just daydreaming. When she see me looking around she believes I'm checking out other girls when it is not the case. I have tried to explain to her my situation but she dismisses my explanation. She also believes I don't care about her even though I do but I find it hard to express my feeling and she is the only person who I have truly loved. I admit that I get frustrated really easily and we have a lot of arguments which start from me getting frustrated. Even though I have tried to show I care for her and love her, she keeps telling me I need to show her, I don't know what to do. I don't want to lose her but I don't know what to do!