why do i feel the pain?

i've never been through relationships but i know even in my dreams how to love and be loved in return but i just can't help myself from thinking why do they always tell me they'd wait for me and will always be by my side when just after i turned my back there they are with someone? many times have i experienced this many times have i cried and i keep wondering is it me or am i just a fool for love? i tried closing my heart to avoid the pains and hurtings but why is it that when i try to open it for someone everything's back like it was before same pain and same hurting same reasons and same memories i'm trying and must forget...