Can't tie a fly fishing knot?

Hello!

I just bought my very first rod. The backling line had already come attached to the fly line (I bought a prepackaged rod).

But I spent the better part of the night trying to tie the fly line to the lead, and didn't even try to tie the lead to the tippet. I tried a blood knot and a nail knot. I used an empty pen tube to thread the end of the string through the knots on the nail knot. Someone told me that would make it easier, and it did, but when the knot is finally tightened, it's really ugly looking and when I tug on it it comes loose real quick.

I'm wondering why it's so hard! I'm 6'9" tall, and my hands are pretty big. Is that why?

I was thinking about going to a pro shop tomorrow and buying another lead line, because the one I was practicing on it pretty much destroyed. Should I ask them to show me?

Thanks!