I need opinions and help on what I should do now with my relationship..

Aight I thought I would go out on a limb here and ask reddit for some help. I have had this girlfriend for over 9 months now and everything was amazing until the past month and I could tell something was off. I know we spent alot of time with eachother but I gave her my 110% and tried to make her as happy as possible. She would sometimes say I was changing to much for her but it was always me. I am just a very kind person. Anyways a week ago she said she needed some space but we werent breaking up. Well I wasn't for it but I wanted to do what she thought best. Well three days ago I found out she has been talking romantically to some guy. I was furious and hurt. She tried to apologize and was saying she was so sorry and stuff and was saying maybe we should just take a break... well i just said I needed to think to myself and went back to my apartment.. well next day she said we needed to talk. Turns out she was so called "lonely" after I left her so she went over to that guys house and slept with him. I snapped and was so dissapointed and angry with her. She hurt me so bad. She was trying to tell me she regretted it so much and that she loved me but we still needed a break..she said she needed to find herself before she could be happy in a relationship. I said fine but you dont know what you could be possibly losing.The weird thing is I still love her and care deeply for her and I dont want to lose her... And now she doesnt even seem like she cares about the pain I am going through. She even blamed me today saying I just wasn't being strong enough and that all the burden she was carrying just made her crack? I don't freaking get it. So what do you all think? I need some good advice....