This is exactly my experience too. It's very similar to the issue I always have with mindfulness practices. I can focus on a particular thought or image, or even just "observe" my inner monologue for the smallest fraction of a second, but then it's right back to the noise of uncontrolled thoughts. Just like you said, I can picture the apple just fine, even in extreme detail, but as soon as I do, it vanishes again and I have to reimagine it anew.

I've likened it to being like a game of Hot Potato my brain unintentionally plays with itself. The same instant I've grasped the potato (or apple in this case) it's tossed away again. It's not hard to catch it again after I toss it, but I can't help but toss it as soon as I do.