I've kinda been in your position before. I thought I was really unattractive as I had spots, didn't think I had a pretty face and didn't know how to dress very femininly like some girls you see. Now, I'm not saying it's perfect now, but I've found a style that suits me, I've learnt to do my hair abit more, I've experimented with make up (just mascara, eyeliner and foundation). I wouldn't consider myself unattractive anymore and though I still get self consious about my looks with people I'm attracted to, things are alot better.
Everyone has there talents and I think some people just naturally know (or at least care/enjoy it so much they take the time to learn) how to dress and style themselves. I am not one of those people and for someone else in thsoe shoes, I'd say it's a learning process and not to expect things to happen instantly.
I'd also say when you feel unattractive you tend to distort reality and see things that aren't there. You have dark hair and green eyes....those are really, really attractive qualities, the type of thing that makes me jealous of other girls!
I think instead of looking in shops (as in my experience that was really intimidating and bewildering when I didn't really know what I was looking for) you should start by looking on the internet and finding outfits for women of your size that aren't too ambitous to pull off. Look around the shop websites for similar items that contribute to those outfits and then try them on in stores to see if they fit. You need to drop this baggy style as it isn't makign you feel good about yourself and selling your qualities. Opt for fitted things.
Also, make up is another thing that is really bewildering. Someone once suggested taking classes as supposedly they do them now. I couldn't be bothered personally but it might be something you're interested in. Like said, I'm only on the basics of eyeliner and mascara so perhaps you should just start off with that and when you feel you've got that technique down enough experiment with more complex things.
Hair, I think, is really important. One of the most important things perhaps. I just sort of got better and better at it through trial and error and chance encounters with helpful people. Think about changing it perhaps to something more stylish and feminine.
Anyway, last comment, everyone's got to start somewhere. It was a process for me that's still going on. Take it in steps and you'll see results