They are ment to ride up your crack, and be there to be hidden. I bet you'll get used to it with time. It's just like boxers for boys. When we're young, we have to wear tighty whities, and when we go to boxers, it's very weird having it hang there, and so on. Give it time, I think thongs are sexy, and many others do too.
It is made so that the back strap rests across your anus and crack. That is normal. But it shouldn't pinch pull or rub. If it does you are either adjusting it wrong or have the wrong size. Also different brands have different dimensions so you might try a different brand.
It's supposed to do that. After a few days it wears off.
I wear a thong all the time, even when I go to the beach or pool
A properly fitting thong doesn’t ride tight up your ass crack – Guys you will never find any underwear or swimwear that comes to comfort and support you get from a properly fitting thong. The problem is most guys think they have a small waist, when they need a medium or large.