How do i let go,Plz i really need advice...?

This is really corny of me to be asking the world how to release from a love i have for someone that destroyed my heart and cheated on me three weeks ago,i guess i can blame myself for my wrong doings but if she wanted to leave me that would of been fine but ,how it all happen went really bad,she was leaving to see her family for her dads b-day and said shed be gone for a week and she and i were like i love you ,have a good time and goodbye and that night she had sex with a guy she just met and then 4 days later told me shes not comin back because she miss home and friends and i kinda had her tell me the truth about the cheating she denied then i kinda thru a lil lie out there like i knew ,i went fishing ,it broke me bad and now i am trying to get over her but i just cant and i even forgave her and the sad thing is we had a apartment,finances together,we have 1 child each and they both look at each other as brother and sister and us as mom and dad so my family is destoyed im still hurting and i still help this girl who broke my heart she left me and she has nothin now and i still send her money and help because she always says well i guess you dont love your son no more, it kills me beause all i want is to be happy and have a family,i go to college, work and come home,she wants to party,and have friends but asked me to take care of her son ,i dont know what to do no more, im sorry i may seem pitiful but really am just seeking advise from maybe someone whos been here or even someone who been the betrayer,i need to find strength to let go but i cant help missing her and our family we had.......