what should i do now? i felt so sad

i quarreled with my BF last week.
because i found one of his daily by accident
all the contents he write in the daily is about he can't forgot his last GF though
they had already borken up.since that girl is his first love and they are be in
together 3 years.so he felt sad when think about her.

after saw this i felt so angry then i asked him whether all the words he writes all
true.he said yes and apologise to me . he said he can't absolutely forgot his
frist love in a short period of time and hope me understand it.but he'll adjust
himself soon for me.now his love is me . he's only have a cherish of memory of that
girl.
though he had explaind to me but i'm still felt angry so i ingored him for a week....till today.this morning.when i buy something with my friends at the street i saw a unexpected scene.he and his last GF, they entered a restaurant together hand in hand...... i was shocked and in emotional collapse.....
i don't know what should i do now.i want to break up with him and scold him loudly
but i can't decided to do this as i really love he.i can't make up my mind.
i treat him with true heart .....i feel i can't put him down.