Why can't I trust my girlfriend? Please help.

MrHopeless
My girlfriend and I have been dating for about 10 months. Allow me to preface this by saying that I am 27 years old and have had 3 serious relationships in the past 10 years, 2 of which I was cheated on. I was also cheated on in my first serious relationship when I was 14. I have never been unfaithful to anyone, and I take cheating very seriously. I actually find the idea of monogamy very sexy...and my biggest fear is being a victim of infidelity, losing the one I love.

I've always had a problem trusting my girlfriends in the past, but for the first time in my life, I really trusted this one because she always wanted to spend time with me, she was always complimenting me, and we made love every single day. Even if she wasn't really in the "mood", she would do it anyway because I could get her there pretty easily. She made me feel very good about myself.

Now, everything has changed. She keeps turning me down when I want to make love, she doesn't ever seem to be excited to see me, and she wants more time alone. I'm a fairly attractive guy, but I keep worrying that she's interested in someone else. She's very pretty and has lots of friends, and it worries me all the time.

As for making love less often, she says that I don't let her "miss it", and she wants to miss it, apparently. We still do it 2-3 times a week, but what concerns me is that it just suddenly dropped. I'm always asking her questions about what she's doing, and it's starting to p*ss her off. If she really is being faithful, then I can understand why. I really want to trust this woman because I love her with all my heart, but every time she's not around me, I think she's doing something she shouldn't be doing. She's never given me a reason to not trust her; it's just that she wants more time alone now and doesn't seem to be attracted to me as much anymore.

She has an 8 year old son and she lives in the same apartment building as I do. We actually work at the same place too, but on different sides of the building. You'd think this would deter me from these thoughts, but it doesn't. I'm constantly thinking up ways that she could possibly be getting away with it.

What the hell is wrong with me? She trusts me and tells me "I KNOW you will never cheat on me" and says that quality is hard to find in a guy, but I don't understand how anyone can "know". Is there anything I can do to trust her again? I don't think she would cheat on me, but I still don't trust her, and I still think about it all the time.
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I also want to add that when I ask her why she seems so disinterested in me lately, she says that she just gets "comfortable" in relationships after a while, and doesn't stress out about not seeing her boyfriend as much. I'm not like that though...
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I forgot to add that she deletes all of her texts and e-mails on her phone pretty much daily. I asked her why she does that and she said she doesn't know, she just likes it like that. Seems odd to me.
Why can't I trust my girlfriend? Please help.
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