No sex drive at all :(

Dear IMs
I am having a real issue with sex. I'm just not interested at all. The problem is not my husband. I absolutely adore him but I'm never in the mood at all. He understands that I'm not in the mood and I think it may be hormonal (although I'm still a couple of years off 40) but he takes it as an indication of how much I love him if we don't have sex at least twice a week (minimum) and gets very insecure. He keeps suggesting lingerie and games to spice things up meanwhile I'm asking him to leave the lights off because I don't want him to see I'm faking it all the time. I don't feel he is being overly demanding (most of the time, although he is a man!)
I can feel in myself that the problem is in me. It's like the switch has been flipped to off and I can't flip it back on. I've mentioned it to my GP who basically said it'll be fine but offered no solution. I've heard of Horney Goats Weed but I've never heard of anyone having real success with it. Nothing that is meant to be an aphrodisiac works. Its been going down hill for the last 6 months but the last few weeks it's just gone. I have no interest at all. I don't want this to effect our marriage and I'd really like to enjoy sex with my husband again. I'll take any and all suggestions. Thank you.