Why dont i trust my boyfriend?

hi ive been with my boyfriend for a year now, and i still dont trust him. we live togeather and i have a little boy who he adores and refers to as hes son but i have so much trouble ive never been cheated on, but the guy iam with left hes ex girlfreind to be with me yet iam petrified he will do the same to me what can i do to sort my head out, everyone at work says we look amazing togeather and they can tell he is besotted with me but iam just ruining it and iam being stupid but these silly thoughts always enter my mind. any ideas? thanks