How can i trust that my girlfriend wont cheat?

im 21 and in my first relationship. ive dated plenty in the past but never settled down until now. a lot of the girls ive encountered in the past have made me weary of the true nature of women, ive had several women cheat on their men with me, leave their men for a few weeks of fun with me until things eventually go sour.. i feel terrible for having been 'that guy', and i think im suffering now because of my past endevors. its difficult for me to trust my gf although shes done nothing to tarnish my trust. please, anyone.. advice?