Help me pack!!

I'm now in full planning mode for South America.  I've printed off all the documents I need and writing up my packing list.  Speaking of which, here it is: (I'm packing for all 4 seasons and 4 weeks)

Clothes

• Thick, warm jacket
• Thin jacket
• Leggings
• 1 cardigan
• 2 long shorts
• 1 skirt?
• 4  long-sleeved tees
• 1 jumper
• 4 short-sleeved tees
• 2 pairs jeans
• 1 pair loose pants
• 4 dresses
• 2 singlets
• Pashmina/scarf/wrap
• 4 pairs sock
• 10 pairs undies
• Rain poncho
• 2 bikinis
• 1 kaftan
• PJs
• Dress for wedding

Footwear

• Nike Frees
• Havaianas
• Ballet flats?
• Shoes for wedding

What do you think?  Too many of something and not enough of something else?  I have a tendency to underpack (yes UNDERpack) and I often end up with not enough clothes to cycle through.  Wondering if one cardigan, one jumper, one thick jacket and one thin jacket is sufficient?  Do I also need a blazer?  Ballet flats or no ballet flats?  Worth bringing a skirt?  Will I wear it?

HELP!