Do you think it is possible to rebuild a relationship where both people have messed up?

I was hurt deeply when my “SO” stepped out on me. There were many lies on their part but she/he does not seem to see it. There was a separation time where she/he was not with me to work threw the issues. After many lies that went on for months I just began to stop caring or believing any thing she/he said any more. Thus I ended up doing the same thing but not becuase I really wanted to. I do not understand why I did what I did and I think I need therapy. I had always been a very faithful person with out any reason for my “SO” to ever suspect me of being unfaithful. I loved her/him so deeply that I never thought this would be possible. I feel so lost now. My “SO” acts as though she/he can’t trust me which is understandable but at the same time I know I had always been the one who was true in our relationship from the beginning. I feel like I have been ****ed over by my “SO” and all “women/men”! I feel so lost and I do not think she/he will ever love me the way I always wanted even more so now.