I recommend vitamin e and more specifically picking up a copy of earl mindell's vitamin bible and looking under sex for foods that will aid in this, unfortunately very common, problem.
I think I will have to disagree with monkeyflower on this one. My husband is 46 and within the past couple of years has experienced the same problem. My own libido has slowed down alot since I turned 40 as well, and let's face it, after 25 years of marriage, it gets kind of, well....."same ole', same ole' if you know what I mean. Don't get me wrong, when the timing is right, it's great, but to me it's just so much work any more. Takes a while to get him erect and sometimes it's only halfway there.
My husband and I are also talking about perhaps some sort of help in that department (Viagra, Cialis, etc.).
I don't really think it's anxiety driven, but just a part of the aging process. I've talked to many people and this seems a common thing in men around this age. My suggestion would be to talk with your GP and try a couple of different things. Can't hurt, right? And personally, my husband would never see a sex therapist, not now, not ever!!
Sorry to disagree monkeyflower, just a different perspective.
Cialis is great... my boyfriend takes it. Cialis helps to keep and or achieve an erection. It doesn't increase desire etc so you have to want one to have one but if you want one it will definately help you get it and keep it. It can last up to 36 hours and is a great confidence booster to help you break the vicious cycle. Talk to your doctor- it is well worth it.
I tend to think this might be a vicious circle. Your wife's lower interest in sex makes you more anxious about having sex, which makes it more difficult to get/maintain an erection, which makes you focus more intently on your erection/sex and makes you come faster, which increases your anxiety... and so on. I think it might be a good idea to see a sex therapist.