Masturbation can't "desensitize" your genitals. The only things that can really do that are certain medications, or some injuries.
Here's the thing in terms of what you're asking: you should be aroused before intercourse if you're going to have it, and while fantasy can certainly accompany that, it can get iffy if it's needed at all times with a partner.
In other words, make sure you are actually attracted to and wanting to be with said partner, and that they arouse you. Anxiety can have a lot to do with things as well, so if you don't trust a partner, or don't really want to be doing what you're doing with them, that is going to have an ill effect on both orgasm and arousal.
Equally important is that you find a way to keep from being frustrated and "reaching" for orgasm. Spectatoring, as it's called, is basically the most common thing that keeps a lot of women from orgasm. Instead, enjoy what you're doing for what it is -- and if it isn't enjoyable on its own, really, don't do it. But the 'as much as I try" makes it clear that you may be your own undoing. No more trying.
Lastly, intercourse alone cannot bring the majority of women in the world to orgasm. So if that activity doesn't cut the proverbial mustard, either combine it with others which do, or do other activities entirely. Any form of sex is not required of anyone, and if something isn't enjoyable for both parties, it's time to find what is.
That's just a start, but those are the most basic things to start thinking about. In terms of being 'aroused enough," though, one does know when one is or isn't aroused, and if you just don't feel you are, it's time to evaluate the whole thing.
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