We've had some great reviews.... Of which this one particularly stands out as matching your needs...
Although ideally you don't want anything which will desensitise her too much. If she is simply becoming overly sensitive after a long play session, that's one thing. To solve this you can discuss different ways of touching her to spread out her pleasure. Rather than focussing all of the attention on penetration, why not play with her clitoris, nipples.... well, her entire body, to ensure one area doesn't get 'fed up' too quickly?
On the other hand, if she's experiencing pain after a while, I would recommend taking a different course of action. Pain is there for a reason and it's usually to tell us when something is wrong or your body needs you to stop doing whatever it is you're doing.
Any sort of lubrication will help stop chaffing and rubbing (and will stop her becoming sensitive too quickly), however, if she is requiring a numbing agent to enjoy sex, it might be worth discussing different positions/thrusting rhythms/pace to ensure she isn't getting sore/worn out. Also, bare in mind that any repetitive motion creates a swelling in the area over time, so she will be getting tighter as your play session continues, which increases the chances of things becoming uncomfortable for her. Not to mention the fact that your erection will grow in length and girth with excitement as you continue. Effectively, you're squeezing an incrementally increasing peg into an incrementally decreasing hole!
Don't forget that no one is designed for hours upon hours of penetration, so if this is the main act you're indulging in, I'm not surprised she's getting worn out. If this doesn't work (and it is pain she's experiencing), I'd recommend a visit to the GP, as sex shouldn't be painful.
And don't forget that with any topical anaesthetic, it only temporarily numbs the area, so any sensation which is removed by application, will return once the effects wear off.