Please help - I know I need to move on but, having trouble moving ped certain things my ex has done to me?

It's hard for me to grasp the fact that someone can spend 7 yrs. with a person, show love & affection for that person, buy beautiful expensive gifts constantly, make plans to marry that person & even announce to friends & family a mth. before ending it of there intentions to marry soon & even buy flowers (something they have done often for no specific reason) the day before while professing there love for you & then 2 wks. after breaking up with you, meet someone else & start professing their love to that person & their wishes to marry her. It almost seems inhuman to me that a person could possibly change their feeling for someone that quickly & project those deep feelings towards the other person. It's been a little over two mths. since they got involved & they appear to be inseparable. Neither one of us has made any contact since. I know it appears that he was cheating on me before ending it but, that is not the case. I have read over & over that rebound relationships very rarely last but, they appear to be going strong. I need to know if it is possible to totally lose love for someone in a matter of days & love someone else in that way soon after. It makes me feel like the last 7 yrs. was nothing but, a lie.If anyone has ever been through anything similar I would appreciate some insight. I am not chasing after my ex or stalking him in anyway but, we do have mutual friends who tell me things & she lives down the street from me so it's hard not to notice that he is there. My friends have also, told me that certain things he was not accepting of in our relationship he is with her which really tears my heart out. How do I get ped all these emotions or am I expecting too much to soon- I want to feel happiness & a purpose in life again!