My girl's sleeping with boss.. again

MY girlfriend has started sleeping with her boss again and I feel like trash.

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This is the third fling she's had with him in 18 months and I'm struggling to understand her.

She comes home at midnight stinking of his sex and cigar smoke. She tells me she's going to an "out-of-town conference" when I know they're slipping away to a hotel for the weekend.

I'm not stupid. I can see the signs. Suddenly she's wearing loads of make-up, fashionable clothes and high heels again.

She's demanding more action in the bedroom from me (probably because she's so sexed up from being with him) and keeps sneaking off to make and take endless phone calls.

I realise I should confront her but I'm exhausted and feel powerless. She knows that I can't stand the thought of going back to square one as a single man.