I went through a lot of the same issues with my DH over the course of about five years after the birth of DD2. I can only speak to my own experience as someone who has come out the other side of this relatively unscathed.
This was my own personal experience and it may not be relevant to anyone else.
At the time it was almost impossible to get dh to talk about what he was feeling and thinking and it was only much later that I actually understood what he was going through.
It was only gradually that I noticed the fall off in frequency and I went through all the feelings of self-doubt that others here have already mentioned.
Since then we have been able to talk about his feelings more openly, although I still haven't got to the bottom of what really caused his lack of interest. However, one thing that I found really enlightening and just hadn't even thought about when we were going through this was that he said that many times he was aware that we were going long periods without DTD but that he was afraid to initiate sex.
Now, before you think he has hang ups about sex, that definitely isn't the case. What he was afraid of was that I would start questioning him afterwards. He was afraid that I would ask him the obvious questions as to why it had taken so long and he didn't know himself why his libido had declined and he was afraid of what my reaction would be if he had no answer for me. This led to a vicious circle where he would feel even worse as time went on as he became even more worried about what I would say or ask him if we were to have sex and so it became even harder for him to initiate sex.
What proved the turning point for us was when I bought a book called The Art of Sensual Massage. We had always been close, despite not having sex and he had always given me back massages and foot massages etc. But this book is about doing massages properly and in a very ''sensual'' way.
He became really good at this. From here I encouraged him to spend more of the massage with his hands where it felt best for me. Although I did need to reassure him that this wasn't me trying to pressurise him into having sex.
From here we moved to much more overtly erotic massages and we got some good ideas from a website called www.hegre-art.co.uk - warning this site has nudity on it - and I would say to DH ''Oh can you massage me like that?''. One thing that I learned about from that site was a vibrator called a ''magic wand'' - have a google of it or look for it on Lovehoney. Oh boy does that thing work.
While we were doing all this DH would sometimes get aroused. Not so much at first, but more often as time went on. I would reach for him and, initially, he would back off thinking that I was pressuring him for sex. But after reassuring him that I wasn't he became a lot more relaxed and our massage sessions gradually became a lot more ''mutual''.
From here it was a gradual transition to including penetrative sex in our massage sessions.
To this day I still don't know what caused my DH to lose his interest in sex during this time, the most I've got from him is about the vicious circle that he felt himself trapped in that I mentioned above.
Well, that was our story. I know everyone is different but I would bet that most DH's want to give their wives pleasure but may feel intimidated that making a move after such a long time would lead to all sorts of questions from you that they don't have answers for.
But they still want to give you pleasure and a full body massage could be a non-threatening way of starting the process