I am 23 my parents have been asking marriage and grandkids I told them I'm doing neither to soften them a little. I know I am gay and I want to tell them but when I'm about to I start questioning myself trying to be sure. Even though I know 100% I AM. they have told me they don't care if I am they just want to know. Why is this so hard.
This is mostly just your fears of coming out speaking. Hell, after I came out to my parents I couldn't think of anything "gay" and thought I was turning straight. Muster up that courage and come out. If you're not comfortable doing it verbally, you can always do it through a letter or an email. It's best to get it out of the way as soon as possible, especially if your parents mentioned that it would not bother them. That will take a load off your shoulders.
I agree with Hefiel above, I came out to my parents a few weeks ago and both before and after I had so much doubt resurface. BUT like you said, I am 100% sure and it was just the finality of coming out to my parents - since they were the ones whose reactions i feared most - that made me doubt everything that I already knew. Knowing that your parents said they'd be accepting is AWESOME. And even mine surprised me If it wasn't hard, it wouldn't be worth it. You seem like you really do want to tell them and are ready to. Just take a few deep breaths and push the words out. I know it is very difficult and you can say the words in your head but for some reason making them into real words seems so hard, but you can do it. You'll feel much better afterwards.
