How can i stop thinkin bout gettin cheated on..even when im not gettin cheated on i seem to think i am..?help?

ok i seem to think every gurl i go wit is cheating on me due to past relationships an alot of pain i had to deal wit..its hard for me now to really trust do to past events an it has messed up alot of my relationships becouse of the way i be thinkin.its hard for me not to think that becouse like i said i been cheated on almost all relationships i been in an dats all i seem to think but i really love the person im with an we had this talk bout this an she ured me she wouldn't due that an she is 3 weeks preg wit my baby..i don't want tha past to effect how i think but it keeps commin back in my head an i think bout it an i keep askin her bout it an she startin to think i don't trust her an we havin this baby this ain't gonna help...yanno so i need help on how to not let tha past effect my future relationship...