@roger,
Yep. The more you try to force her, the more resistant she'll be. She's got to convince herself that you're a safe place she can trust.
Just let her jump up on your lap on her own time. Once there, let her get comfy and relaxed without petting her. She may not stay there long at first. You could get a few kitty treats to hand feed her while she's on your lap. After a few times, try giving her an occasional head scratch and then leave her alone. She'll either respond to it or jump down. Take her lead from that.
If, by the time cold weather arrives and she's still not lap sitting, keep the temp cool in your house for awhile and she'll probably be more eager for the warmth of a lap.