struggling with trust

Before i explain things ill give everyone that doesnt know my story a quick insight.
MY gf cheated on me about 6 weeks ago,she kissed this guy whom she works with on a night out,and told me she had feelings for him,since this event we have had up and downs but were are trying.
Anyway,my trust for her is shot to sh!t which is understandable but the crazy thing is,i do trust her in some cases but just not with this guy. When were together i do feel pretty normal again with her but when we aint together i feel crappy,like when we text and her replies and short or she takes ages to reply my head goes crazy,but its only thinking that shes texting him or something.
She is going to the seaside at the weekend with people from work and he was going,she said he isnt going now and i believe that he isnt but i suppose something in me doesnt believe that, ive told her everyday for the past few days that im worried etc which i thought was the best thing to do to get it off my chest. Tonight she said its doing her head in cos i keeep going trough the same stuff,i cant help it though,i believe honesty is the best policy hence why ive told her whats going through my head.
Im trying to do things to take my mind off it all but its hard,ive even got myself a job interview tomorrow which will be my 1st job in over 10 years (ive been ill thats why i havent worked) but im still gonna struggle so much this weekend and i really dont know what to do or think.
Then again its just not this weekend that bothers me,what can i do? she is trying harder to make this relationship work but im struggling to get past my trust issues :(

I'm just one guy talking here, but if I was in a relationship with a girl who had feelings for a guy she works with, had kissed him and who was potentially hanging around with him over weekends away then I would have trust issues too. I don't think its a situation where you have to just accept it and that's that. I think you're right to voice your hurt and I think you're right to communicate your concerns, given what happened. How would she feel if the shoe was on the other foot?

Honestly, I'd put up with this for a while if the relationship is really serious and I would ask her to work at it with you. If you're still feeling this way about her after a few months and she continues to give you reason to doubt then I would end it.