How do I let go?

I was with a woman for 3 years we have a son together now. I have been open honest and loyal to her the entire time. She however has cheated on me multiple times and has lied to me more. She has done me dirty in so many ways it's outrageous that I still have feelings for her. I will admit I haven't always reacted in a compassionate or understanding way when I found out something that hurt my heart but I have ALWAYS been supportive and comforted her when she felt her life was unraveling in areas that didn't pertain to me. She told me many times she wanted to marry me my response was even though we don't have rings I feel we are already married. She told me many times before we conceived our son that she wanted a baby with me I told her I didn't think we were ready. Now that we have our child and we are living separately she doesn't want to spend time with our baby (monkey man) or be in a romantic relationship with me. More than that she is trying to prevent ME from spending time with our son because of her parents spite for me. I always told her when I would find out she was spending nights at another man's house or giving oral to some random man that if she didn't want to be in a relationship with me just tell me however now that she is openly admitting it to me now it just seems to much to handle. I want so desperately to have a family that is loyal to one another and supportive to each other's needs. Our son is my world and I just wish she'd see him the same way I do and I wish she would see me like I see her... As an angel sent from our Lord and as a human that has made mistakes. I know I can't change how she thinks or feels I just need to know what to do for myself to cut ties with my feelings for her so I can effectively and efficiently move on with my life.
How do I let go?
How do I let go?
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