Wife's lack of intimacy is driving me crazy! What can I do?

Over the past few years my wife has ceased to initiate intimacy and when I try to initiate she rejects me 95% of the time using excuse that she is too tired or makes comments like "all you think about is sex". We've been married 34 years and my sex drive is just as strong as it was when we got married but apparently she's lost hers. I've talke to her about it and get nowhere.

I bought her a nice Valentine's day present thinking that might start something but it didn't. Oh she liked what I got her but it didn't do anything for her affection. Her birthday is coming up and I know I'm heading for the same problem.

I feel like we are just roomates anymore raising the last two of our six kids at home still. When I read posting from women that they have the opposite situation and can't get their husband's to have sex and I think "why isn't my wife like these women?"

Any ideas out there?