Sex has become a chore

Been with my fiance for 8 years. Sex has generally been good (until recently). Id say 3 or 4 times a week was a norm. Starting in late June or early July she lost her drive for it. Sometimes it happens, you go a week or almost two without it then you get back into it. But this time I get it only after working her into the mood. I know she doesn't want to rush into it but I've had her in bed, things are hot and heavy like we are a couple teenagers, and then bam, 'I'm not really in the mood'. 2 weeks isn't a long wait I know, but when I could go 7 times a week or more it feels like an eternity. When we do finally get around to having sex I can tell it is more of a chore to her than anything. Id rather do it myself in that case (which I do often, sex or no sex). We have talked about and she says she doesn't know why. She isn't on any meds including birth control. She doesn't handle finances so she isn't stressed from that, and even then we do just fine. If it was depression than I don't know why last summer was just fine (friend died, sex still went on like nothing was wrong). She says it isn't me and I do believe her. And I know she can't be getting it on the side unless she leaves work early to do so. I've read a bunch of the low libido threads but can't find a situation or tip that may help. I told her if there is anything she wants to try that I would be willing (I'm a freak, she is kind of a plain jane in the sac, I thought it may spark something for her). She even offered to allow me to fool around with someone else. While the thought is tempting, I wouldn't even know where to begin with that seeing as how I am a social recluse. More important is it has been almost 8 years (Oct 30th) and I've never strayed, through good and bad, and would rather not. I'm not close to considering that offer yet, but at some point I'm going to become quite frustrated with the situation. I know a little is better than none, but it just doesn't do it for me. Sorry if I left any other important details out, will answer any questions anyone has if I can.

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