Long post incoming!
I think the key to packing light is making sure everything matches and you can make items do double duty. A camisole that can be worn as a top during the day but double as a swim suit top as well. That sort of thing. Sticking to a single color scheme also makes life easier.
I spend 10 weeks every year in Scotland. I only bring a single roll on suitcase and a purse or backpack. I'll give you an example of what I bring. (This is in winter, mind you, but you can adapt this to summer)
1 pair of jeans (dark) You could trade this for shorts.
1 pair of tights (black) Eliminate this and add swim suit.
1 sold color dress - midlength (can be dressed up or down using accessories)
2 cardigans (I would substitute a nice silky robe or pjs for this in summer or have 1 thin cardigan for breezy weather)
1 fuzzy/cozy sweater (Good for lounging in cold weather) I would sub this for a second dress in summer.
3 Shirts. One in a trendy style for dressing up, one that you can layer over a camisole or wear alone (think collared- button up) and one plain tee shirt.
2 camisoles (one for layering/day time and one for bed)
Scarves. Generally same color scheme but different patterns. One always a solid. (Scarves are my weakness. I always bring too many and buy even more.) I would suggest at least one scarf made from light fabric. Uses for scarves are endless. You can cover your shoulders when you are hot or cold, wear as a skirt, or beach cover up, add it to a dress for night time, wear over your head as a sun shield...etc.
1 pair of boots for every day use. I usually go with riding style boots because they are easy to dress up and down. You could trade these for comfortable walking shoes for summer.
1 pair of solid color flats. These are used for nights out. (I add a fuzzy insole and they double as slippers)
2 bras (one light, one dark, both easy to wash material)
As many pair of underwear as you feel comfortable taking. I try to cram in as many as possible. (This is my 'just in case' item)
2 pairs of socks.
Now, this is my basic list. I think if I need it, I can always buy it there. No big deal.
I try to keep everything within the same 3 colors. So I don't have to worry too much about what goes with what. (This year was black, tan and red for instance)
I also know that I will probably get tired of wearing the same thing over and over and seeing other women in trendy clothes makes me want to drop the credit card at the nearest mall so, I have a rule about shopping: 1 in, 1 out.
If I buy something new, it must be followed by donating something I'm tired of.
Accessories can make the most simple items really sparkle. I like to buy my accessories when I arrive at my destination. Kind of serves as my personal souvenir shopping.
As far as purses/bags go, I would definitely bring your backpack. If you feel like having your stuff beind you feels unsafe, trade it for a cross body bag. Personally I think it should be big enough to hold a camera, wallet (personal docs), hat, scarf and room for any small purchases made that day.
And as for being a nervous traveller, I like to bring a photo of a loved one (my cat!) to look at when I feel down and a book of puzzles to solve to pass the time.