Still not over divorce?

ok i married and had a baby at 22 years of age and was the happiest girl alive and was for a couple of years, in fact never knew life could be so good, however it all went pear shaped. my husband supported the family and is an excellent father, but we ended up divorcing when my son was 3 and then got back together for a bit but it was never the same and parted for good. my life has not been happy since and i regret ever deciding to part as i feel ive lost the love of my life, this is now over 5 years ago which i no is a long time but i still have problems dealing with this. the reason we separated is because my husband was having with this young girl who was infatuated with him, i knew he loved me and would never have left me for her but i went off after having my child and i spose he needed it somewhere. i still blame myself for not being a good enough wife but when i found out about this i just couldnt live with it, i know some women can and brush it under the carpet and i just wish now i could have forgiven him. now they are married and very happy together and expecting a baby and it breaks to heart as it feels like she has taken what was mine and if i hadnt been selfish for neglecting him it wouldnt have happened. i havent been happy since the break up and feel like i never will and long to have the family i once had back but i couldnt cope with her always in the background. why do i feel so worthless and guilty for letting this happen, he would have given me everything, was i so wrong to be so angy and hurt for what he had done? was i right to leave him cos of it?/ it just feels they have the perfect life now and i basically allowed to have it as i was the one who left?? please help