I think you have had some excellent responses, there's not much I can add.

Rose makes a good point about the nature of long term 'love', I don't know it's ever easy, and certainly the nature of that love evolves over time. You write you may love your wife but not be 'in love' with her, that is just as much to do with your own definitions of what 'love' and 'in love' is. My own experience is that 'in love' is a relatively (a few years) of mad hormone fuelled emotion, but it does peter out - that is why it's not recommended to commit to marriage too quickly - what you are left with is hopefully a mature love. I regard you saying you love her as a positive sign among the difficulties you are having.