I've just returned from Phuket and had the time of my life-I'm sure you'll love it. I went with 3 friends but will be returning in 2 months solo and intend to stay a few months.
I wouldn't worry about going by yourself as friends are easily made. My advice on what to take is-as little as possible! On my return I will be listening to my own advice! I thought I hadn't taken much but I actually needed far less. What I will bring next time:
Swimmers vest tops x 4 little skirts x 2 shorts x 1 flip flps undies x 3
I know this seems very little but laundry facilities are everywhere and you can buy stuff out there cheap. Think of all that extra room in your rack sack for things you want to bring back with you on your return!