I really messed up

My GF and I dont got a lot if personal time together, so we start going to a friend of mine house to be alone, however he(my friend) was still there always in his room and my dumb ass dont think dont think to tell my gf tell just late night after we had all would been over there twice and we were sexually active there too. For reasons Im not going to go into here, I dont see him being there as a problem but once i got my head out my ass i knew i had to tell her, now she says she needs time and space which I more than understand, i just feel so lost without her and I dont know what to do now.

To put it simply:Me and my gf have been having sex at a the house of a friend of mine, i do not think to tell my gf until we had been there twice, and when i told her she said she need some time and space and now i dont know what to do