TASTING MY OWN MEDICINE, need genuine brutal feedback:

Me and my current girlfriend started dating after cheating each other partner ,basically started with soft flirting because of various issues in previous relationship and ended them to get together. Now 1 year later , i found her flirting with some another guy (initial honeymoon phase) , i feel guilty of checking her phone. I feel too connected with her but we had so many issues throughout the last one year. She used to compare me with her ex which demotivated in this relationship alot but we worked on it, now comes the new guy which is again demotivating for me Her needs must not be fulfilled with me and she must be lacking emotional support, i am trying too hard to provide her all that but failing every-time because my mind is full of thoughts regarding other boys in her life and i loose focus What should i do?? Last time i was the one who was taking her previous relationship issues now someone else is there to listen my shit