Best to first desensitize her in general , basically teaching that touching all over the body is OK and there is nothing to fear before you try anything sexual. Play with her , cuddle, give her belly rubs and be gentle. Use a friendly and happy voice to let her know your intention. Remember dogs are masters of reading body language.
As you play with her be sure to touch all over, head, belly, legs, paws, work your way down to her lower belly between her back legs and gently caress. Do this when she's on her back and you are playing. Don't stay in one spot too long. Then give it a break. Do this over days, weeks, months, etc. Once in awhile also pay attention to her rear, just brush over her cookie and ass. Don't draw too much attention or be too aggressive. As time goes on make it a routine where you always touch her there are part of play and being affectionate. She will learn you are not there to hurt her and will realize it kinda actually feels good. Then as time goes on in heat, she should be receptive to more stimulation of her cookie and keep on with the bitchsex guide.
If this does not work over trying for awhile, sometimes you just have to accept she doesn't want that kind of relationship no matter how much it would be upsetting. Keep loving her as you always have in that case.
She is a good girl.