I need relationship advice?

Me and my ex girlfriens have been together for 4 years and we were in love till recently things were just falling apart. Ive noticed signs like her not being really interested in me anymore and her lies in april i found a secret account she had were she texted and flirted with other guys some exs and i confronted her about it and things got outta hand and i tried to look past it between all we have been through and through her lies about the account i stayed with her til 2 weeks ago i found messages betweem her and another guy where she said she had tto lie to me to get out and see friends because i get mad over things when that was never the case but her exs and guys she hung around with likes her. And she found this out when we broke up and told them she was single and they poured there heart out. But what really destroyed me is a week ago i asked her if i could come over to see her and i had a gut feeling by her telling me no something was up we had been living together for 2 years and when i went over there was another guy she had known for two weeks and i figured out today after asking so many times and her lying that she slept with him and thet were both f•cked up and she has been lying about where she has been and who she was with but she tells me she wants to work things out and I keep going back to someone I'm madly in love with even after what happened... I don't know what I should do try and get past everything try and talk to her to see what the problem is or just move on but its just so hard.... please help a broken mans heart to figure out what I need to do. To say I still wanted to be with her what process should I go through? And to forgive and forget her how should I do that? Thanks you guys