Doubts, worries and trust issues

[epladdict]

Four months ago I abruptly began a relationship with a coworker whom had been dating someone for the prior six months. After she asked me out to coffee, she made me aware of how she'd felt since she first saw me. We continued to see each other and eventually I was given an ultimatum - "You can kiss me right now or I can take you home." I had heard about how poor her relationship was with her boyfriend each time we hung out, so despite my feelings towards cheating and being even a tiny part of the reason for others breaking up, I did it. I've regretted it ever since. This is just a precursor, though, and I suppose doesn't have much pertinence to what I really joined to vent about.
Six or seven days into our relationship - that is, after I asked her to be my girlfriend - I came down with a severe sinus infection and wasn't up for the drive out to her house to stay the night (her house being the place I've slept every night, save for this instance). Two or three days later, she nonchalantly informed me that a friend of hers had stayed the night with her - in her bed - while I was away. This friend happened to be the very same one that my gf had a crush on "about a year ago." The kicker is that this friend of hers has ALWAYS had a crush on her. Now, my girlfriend is bisexual and this friend is a homosexual female. In my opinion, that makes absolutely no difference as I feel just as threatened as if it were a guy, given the circumstances and sexual orientations. She assured me nothing had happened, and that she was uncomfortable being in her house alone at night because she "lives in a bad area." Despite me communicating how it made me feel and how astonished I was that she wouldn't think I would have a problem with doing this, she failed to sympathize and simply promised never to do it again.
It's been very difficult for me to put this behind me, knowing that she has admittedly cheated on every bf she's had in the past. I don't know what I'm expecting to hear, or if there is a universal solution or "right" way to handle this situation. I recently moved back home and have been spending the night here rather than at her house. I find it difficult to trust her and not knowing if she's in bed alone each night is eating away at me.

[dewilliams2]

Well if you knew she had a history of cheating then you should have known you could get cheated on...although she said nothing happened. If you can't believe her then your relationship won't work. You have to be able to trust your partner...so I say either come to a point where you feel you believe her or where you know you can't and at which point it is time to leave her.

[Gigabitch]

Your girlfriend doesn't like to sleep alone, does she? It's hard to trust a person like that.

[QueenofCorona]

She seems kind of scandalous and needy. What are her good qualities?

Let her know that is unacceptable behavior and it won't fly going forward. Get her an alarm system or a dog if she hates being left alone at night.