@collector Perfect---you're doing it man!
I think this tells us a lot and will help everyone help you. In some sense you are right: it is a "simple" drawing. Therefore, 8 tries at almost an hour each simply tells me that you are still solidly in the practice stage--which, obviously, is where everyone starts!
The good news is that if you continue doing things like this you will move out of the practice stage and into the next.
The GREAT news is that in order to do that you simply have to keep drawing things--which is great because we love doing art right?
Feel free to do a color version but honestly, if it's overwhelming for you then at this point I'd say stick to working on studying drawings.
Will Terry has a ton more "reference" material out there on the web and, of course, there are others.
Maybe start a sketch thread and post your studies with the original on one side and your copy on the other. Do a bunch of those. Like a lot.
And then a few more. Everyone is different but it'd be good to get it down to where you could sketch that frog in...I don't know...5 - 10 min*?
*I'd like to hear from others here if that's a reasonable time for that piece of art. Obviously, if you are coming up with that type of drawing from your head it might take a lot longer. Sometimes I think my sketch phase is as long as my painting phase--I'm constantly erasing and starting over. I can spend hours on an initial sketch. Again, that's when I'm creating the scene/character from scratch.