Wheathr i should trust her ...?

i m in love, our relation is arnd 4 months old,, started in end of sept 2009 ,,shes alredy married but due to som reasns dont hav any relatin vth her huby by lst 3 yrs,, i am ready to accept herin all conditions.. i want to marry her,,mean while i ask her whethr she had any other relations besides her husband after they seperated and before i come into her life,,,,,she told me abt one persons..she told me that shes attracted with one guy,,but that didnt work out... thats it. nothing else happend.. ( before asking her abt her past.. i told her all abt my past relations and affairs ). yst de only i came to know i came to know abt the one more person from outside. she didnt told me abt him.. this person is known to me...someone told me that ..she had an affair with this person..and she refused him because i found her...i also came to know that many people saw them with eachother. i am surprised to know that...as my girlfriend didnt tell me this. As i knows that person verywell..i decided to check his mobile...i got his mobile and checked it ..i found one sms of girlfriend in his mobile..of same type as she sends me..sended in month of july 2009,,,its shocking for me...as my girlfrnd ddnt told me abt this...as i decided to bring her in my life..i think i should ask clearly everything to her...so i decided to ask her abt this...yesde evening we met ..i asked her..abt him..and her sms in his mobile..and also asked why she ddnt told me abt him....but....she refused...shes saying that i m lying ..she did never sent any sms to him..i told her that i saw that sms and then only i m asking her..but shes denies.. i ddnt say anything then..she ask me wheather i trust her or not..if i trust her then she told me that shes telling truth,,that she dont have any relation with him...but i am still confused as i saw her sms and why shes lying vth me...after yestde meeting i m unable to think any thing else..i talk with her two three times,,,but alwes shes dening and ask me wheather i trust her not..i trust her...but shes lying vth me...i heard abt her affair from some else..i dont trust on that..but when i saw hers sms ..then only quest. came into my mind....yet i am unable to decide what to do..our marriag arrengments are goin on ...i rely dont ...my brain s not working..what should i do...