Begging for advice 👏🏼

Joy
Hi everyone, I ask you all not to judge please. 
The story goes like this:
I'm in love and considering marrying a man a previous friend of mine "dated". (For less than a week while we were all on vacations together.) 
 
I can't avoid feeling like I'm a terrible person, even tho (not as an excuse) this girl sleeps with every guy it comes her way. ( not judging) but the fact that me and her were very close a while back makes me think that I'm bad.  Am I? 
Now, I love this guy to the sun and back, I'm moving in with him and we're talking marriage and this whole situation makes me be very insecure about it. I just need some advice on what to do and feel, I'm going craaaaaaazy!!!