I cant move on, please help?

my fiancee left me when i was 6 months pregnant with our planned son 13 months ago, i cant get over it. he abandoned us and left for a woman 13 years older than me with 2 kids of her own and has got everything lives in a stunning home with land etc. im consumed with jealousy and cant get them out of my head , how could he do this to us? he doesnt pay toward him or see him (his choices, cant get money, long story tried), and all i think of is them having an easy happy life while im struggling to keep a roof over our heads paying a mortgage which he also left us with. people stay move on, how on earth can i do that when im eaten up with bitterness and i dont want to be i just want to let it go but its there all the time and i cant get rid of it as i cannot understand why hes done what hes done as ive never had any closure and i keep thinking i couldnt offer him enough as she can offer him so much more than me. i wouldnt even know where to start on the dating front as that frightens me to death and i have no confidence left as he left me broken hearted. will this ever ever go away? will i ever get over the terrible things hes done. and to top it all off they live local and i see her in my town when im pushing my son in his buggy and she looks at me and smirks just like im a piece of s**t on her shoe.like im nothing. im 32 hes 36 and shes 45. this has broken me its not like i can get a life as i have an 11 month old baby so naturally im in most of the time looking after him, apart from when im working and hes in the local creche. please help me with my thoughts.