I'm afraid to sit on his face!?

I've sat on my man's face once during 69 for about 2 seconds while I was Tipsy and then decided that I was too nervous. He's eaten me out many times and it's amazing. But sitting on his face or 69 just seems a little scarier since his face is like up in my butt.. speaking of which he's now been in my butt once haha but it's not like it was up in his face.. I guess I'm looking for pointers on how to get over that and also how not to crush him and make it a good experience for both of us. I shave both areas.. but I feel like my butt hole itself is dark and weird.. is there anything I can do to change this or is it something you just get over? I know he doesn't care and he wants to do it so bad.. he says it's extremely sexy.. I just can't get it outta my head. I also shake alot when he eats me.. any tips! Thanks so much.. I think it'll open the door for other stuff like reverse cowgirl (tips or experiences for this are also appreciated)which we haven't tried either.