1) Stop selling trying to sell this (fruit) to me.

Your sentence would also work, it's just slightly less natural. 'fruit' would typically be left out in everyday speech.

Have you ever caughtseen me buying any other kind of fruit?

I only eat apples solely.

2) I think if one of my family members were a millionaire, I'd have known that.

3) I thought if one of my family wasmembers were a millionaire, I would know that, but it turneds out they've been keeping it a secret from me all these years!

Again, your sentence is good, my corrections just add a little bit of clarity/natural-ness

There' are different degrees of grey between them.

There're isn't common

5) I've been practising this same movement for hundreds of hours, yet I'm faileding to see any progress.

1) Stop selling it to me.

More common:
• Stop trying to sell it to me.
• Stop trying to get me to buy it.

I eat apples solely.

Suggestion: I only eat apples.

2) I think if one of my family were a millionaire, I'd have known that.

3) I thought if one of my family was a millionaire, I would have known that, but it turneds out they've been keeping it from me all these years!

It’s very difficult to correct this without knowing the full context, including how long ago you discovered they’d been keeping it from you. The above correction assumes you just recently discovered the deception.

The following is also possible, if you discovered the deception a while ago:
“I thought if one of my family were a millionaire, I would have known that, but it turned out they'd been keeping it from me all those years!”

They think everything is either white or blackblack or white.

We always say “black or white” in this order.

There're different degreshades of grey between them.

I’m having trouble imagining a context where “between them” would sound natural in your sentence. What does “them” refer to?

My suggestion:
There’re also many shades of grey.

5) I've been practiscing this same movement for hundreds of hours, yet I’ve failed to see any progress.

We spell it “practicing” in American English.

You would be frustrated too if the bootyou were oin the other footmy shoes.

I’m having trouble imagining a context where “if the boot (or shoe) were on the other foot” would sound natural here. The standard definition of this idiom is: “The roles of people in a situation have been reversed, such that the advantage has shifted to a party which was previously disadvantaged.” The last part is important: this idiom is used when someone who was weak now has power.