Why im so? Knowingly , why i'm so?

Is it true that a depressed person always think wrong , I love a girl so much and she too and I know she is very loyal but there was somthng happened that made me suspecting everything , it was not that big issue but still I suspect her in every case , and my study is also effected , I plan very much but can't do anything , I want to faith her but suspecting thoughts are every where , she just tell me a lie about a guy ,she just used to talk him not more than that but why she lied , this thought kills me , I too know that it is not the big issue but still I can't control my self fighting her , anger her , abusing her bcz I think she hurt my feelings ,yes this is the thing Now I'm helpless with the thoughts , I want to love but generally things go wrong every time For study I used to denial situation and my father don't understand me , I thought of worst case scenario Sometimes I find that I want to quarrel ,anger with her knowingly , Why I'm so ?? Suddenly I loose my temperament