What should i do? my girlfrend has lied to me?

i am with my girlfrend 6 months bout 3 months ago i was goin honme from her house and her frend came up she told me he was gay and i got all paraniod tinkin wen they were iunside doin somethin,burt i dont think they did as he left and i went home now i went away for the nite with her sat nite and i got up yesterday and for sum reason looked tru her fone and saw a message to her best mate tellin her she is with me but likes some1 else,i confronted her had a huge fight,she said i shudnt of lookm tru her fone fair enough i shudnt of i was wrong then she told me that no her frend wasnt gay at all she told me that cos she thought i wudnt like her hangin with him othewrwise,she also told me they were **** buds for about a yr and she had feeling for him when they werre,but they work together so nothin could come of it,now this is the guy she had told me was gay and she swore she was never with,she said they have a history but she swares that since im with her she wasnt with him r nebody,and shes sorry n she loves me dont wanna loose me etc,im just wonderin whgat to do my hgead wrecked i do believe her that she wasnt with him since we together cos im with her everynite bar 1 r 2 amunth,but still thats a big fkn lie she told me,now my mate said dont dump her and i dont wanna but how can i tyrust her?