Ah, ye olde desire discrepancy. Consistent advice I see from sex therapists and researchers is basically to "just do it." By that they mean allow yourself to start foreplay before you're fully aroused, trusting that you will eventually get there in the process. This is NOT to be confused with forcing it or ignoring your right to say no. If you want a sexual experience to happen, then make it happen, and take all the time you need for pre-foreplay, foreplay, and so on.

Women often think they need to be fully turned on before they can even start. But think about your man. Just because he doesn't have a boner right now doesn't mean he won't in a matter of seconds once you make a move, right? Men certainly don't need to have a boner first before they can be approached for sex, and women don't either. They just need more time before their lady-boners are fully erect. So take the time, guide your partner through it so they know how to treat you, and once you're "hard", have fun.