breakup advice

My partner of 4 years and i have just split up because of an arguement we had a few nights ago (a guy came up to me in a club and flirted, i told him i was taken and walked away.) We have a 1 year old and i just feel so shit, i love that man so much but he really is an asshole to me all the time. Hes cheated, flirts, calls me names, ignores me on purpose. Hes been such a big part of my life for so long, its just lately hes been so shady, going to parties after work, condoms going missing, hes barely touched me in weeks. Why do i feel so bad that ita over when the things he did made me so sad.