you're fucked mate. the seed of doubt has been planted in your mind, and that's all it takes. Probably a shit batch, bin those and get another lot.
Also order some 5mg cialis, take 1 a day, and forget fucking around trying to time a viagra right. After a week of those you'll be back to the amount/strength of erections you had in your 20's
Cut out caffeine, lower your sugar intake as well, and get the sleep your body tells you that you need these days. It's still most likely mental, but the placebo effect of getting that stuff done as well will likely help bundles.
At a pinch, go down the doctors and get a prescription for viagra (now generic, forget the name), get the 100mg so you can take half and get double for your money, and those should last you until you get your other ones etc