How to trust my gf again?

i have been with my gf for almost 1 year and 2 months. we broke up after i found out she had been emailing an old friend from highschool. it's not the fact that she had been emailing him, but she kept it from me for 3 months. what i dont understand is we are both VERY jealous people and we agreed to never keep anything from eachother, let alone talk to guys/girls without letting eachother know. she left her login information on my computer on accident, and i swear i had suspiciions because of how distant she was being with me. then i just had to look for myself. behold, there were emails that went back 3 months. when i confronted her, she totally denied it and said i was crazy and it was all in my head and she would never do anything like that to me. then i printed out the emails to prove to her, but by that time, we argued so much that she left me because i didn't believe her. we broke up and after a week, she called my work because i changed my number so she wouldnt be able to get a hold of me. she cried and cried and told me how sorry she was and she was wrong. she said she left me in the moment because she was so mad that all we did was argue, but when she realized that she was wrong for keeping things from me, she couldnt live with herself without apologizing. i took her back. my problem now is trusting her. she said she had a huge reality check and she would never do anything to hurt me or keep anything from me again. i hate that im still not completely over this, but how can i get over it and trust her again? i want to believe she will never do it again. she owned up to a lot of things that i didnt even know about. but now, im paranoid she'll do it again. she said she learned her lesson and she never wants to lose me again. she now tells me if a guy even says hi or talks to her...like she said someone she knew from highschool came into her work and she just wanted me to know because i told her to never keep anything from me again...what if i can never get over it? living with doubt is such an uncomfortable feeling ever....what do i do? :(