More poncho anxiety

Okay, I need someone to talk me down off this (very low) ledge of knitting anxiety.
I am making the Artyarns Poncho P53. And I'm feeling like the sizing is just going to be way, way off. I've put so much energy into this already, and I'm really worried that I'm going to finish it and hate it, or worse, be unable to wear it! So, the bummers so far:

1. The actual yarn (Supermerino 108) is an entirely different color scheme than their picture of it (the picture on the pattern is worked in the same yarn as I am using). It's much, much darker. Pretty, but not at all what I was expecting (and those who know me, know I don't do well with unexpected differences).

2. The pattern isn't going perfectly. You make an initial diamond, and you can size it however you want it - I took their suggestion to make sure it covered my back. Then you work 3 diamonds off of it (4 sections total). I have worked 2 of the 3, and I am convinced that it's going to be HUGE. I am worried that the neckline is going to end up so big that there's no way it'll stay on my shoulders.

3. My diamond #2 and #3 and different sizes and I have no idea how it happened. I have an entire extra pattern set (6 rows, one of which is dropped YOs, so big) in diamond #3. So it's an inch or so taller. How big a deal is this? I don't know how to fix it (other than frog the whole thing and start over).

I am feeling really bummed and discouraged, and I'm not trusting that it's going to work out. If it's really too big, the only option is to pull the whole thing out and do it over, and I think I might cry. I love the pattern, but I'm having a hard time seeing how it's going to work.