End of year angst

I find the end of the year a hard slog. The weather heats up to a point where it is unpleasant to be outside between 10am and 6pm, and I can only brace myself for the next 3 months of the same. Then there's the fun of being single again, and floundering in a deluge of happy couple festive advertising. And then the forced interaction with family members who have little sense of personal boundaries, no idea of hygiene, and an instinct for saying the worst possible thing at the worst possible time.
Every year I make the same New Years resolution - a quote from Pride and Prejudice.
"I am only resolved to act in a manner which will constitute my own happiness, without reference to you, or any person so wholly unconnected with me"
And every year it falls down. I take the words and actions of others at a higher value than my own experience and instinct. Why? When my sister speaks condescendingly to me, making me feel like an ignorant failure, why do I take that to heart? When she rummages through my cupboards, why don't I speak up for myself, and tell her to back off?
Seems like as much as I try to grow and change, they don't want to have to change the mental box that I live in in their minds. So they treat me the way they always have, and I allow myself to slip back into the groove of how I used to behave - uncertain, colourless, and weak.
But that's not who I want to be - I want to be someone who laughs loud, and doesn't care if she sounds silly. Someone who is strong, and sexy, and confident in her own ability. Someone who values herself.