As I always say -- and I will say it again:
Communication is key.
You need to sit down with her outside of the bedroom and have a long and frank talk about your sex life. No interruptions; have somebody watch the kiddo for the afternoon.
I'd start out with "Honey, I love to have sex with you; how do you suggest we be red-hot together again, like we used to be?" and let her take the floor.
She owes it to you to make sure you are happy in all areas of your relationship and that includes making sure you are happy sexually.
She needs to meet you halfway on what you are asking for --- once or even twice a week for sex would be fabulous. And once or twice a week for intercourse would be an excellent amount, considering you have been together for nearly 2 decades.:)
To be truthful, something is flakey here with her. I am guessing you are in your early 40s - late 40s or maybe even your early 50s. I don't think this is a perimenopause thing or a menopause thing where her hormones are off -- she's acting like a child; really, she rolls her eyes and walks away? Brilliant. That's being a kid about a sensitive subject, not a reasonable full grown mature woman.
What do you mean by "on and off for 19 years"? did you break up along the line and separate for awhile and then reconcilliate? or have the breakups been more than once or twice? If they have, that might be a clue to what's going on here.
Bottom line: she needs to be a happy and willing sexual partner. If after talking to her and telling her you have needs, and she won't follow through, rethink being with her for the long run. You did not marry to eventually wind up married to a cold fish who decided that the bedroom door is closed for good.
She may simply have decided no more sex with you, for only reasons she can name. It stinks but it happens.
What about you and your appearance? have you gained or lost a lot of weight? do you keep yourself well groomed? Do you shower regularly? do you stay active? are your teeth in good shape? Do you smoke to excess? Do you ensure that every time you have sex with her it's just not the same ole in and out where it's boom, yer done in 20 minutes, no foreplay?
If you look fine, smell fine and your physique is fine and you've been more than adequate in the bedroom, something else is amiss here, on her end.
It could be possible she's already emotionally checked out of the marriage and/or is having an affair. If she's done one or the other or both, I suggest you do yourself a favor and leave; there is no sense in pursuing a marriage when the other person's letting you know loud and clear the show is over.
It also may be a possibility that this relationship is simply over and it's run its course.
If that is the case, she needs to woman up and tell you what the deal is. She shouldn't be leading you on and letting you be the foot rubber, compliment giver and all around gladhand good time Charlie: you want a wife and a partner, not some little 10 year old who rolls her eyes and will not talk abou tthings like an adult.
Sorry for you troubles. Let us know what happens. GL.
ETA: If you do not wish to call it a day over this, and she refuses to have anything to do with you sexually, let her give you the option of pursuing an open relationship. Yep; she stays married to you and no sex on her behalf; you get to go out and pursue sex with whoever you want to pursue it with.
I don't know if this solution floats your boat.
What she's also showing you is a dire lack of respect. She apparently doesn't care about your happiness in the bedroom and that, to me, means she sure doesn't respect what you want or give a hang about your needs or wants or what you wish to do.
If it turns out she's having an affair, call this quits. As we always say on these boards adultery is a dealbreaker.