When I say don’t limit yourself to colours or styles that are meant to be slimming, it is because of what you just described - getting in to a rut thinking you’re limited in what you can wear for one reason or another.
I think when people ask how to change any aspect of how they dress my advice is always going to be the same: Go out to stores and make a day of trying things on, don’t go with the aim to buy a million things, just go to work out the basics, like what styles, cuts and colours you like. It’s so easy to buy things online these days, which is great of course, but it might mean that not many people take the time to look around and tailor a look or trend particularity to themselves any more. If you have a friend or relative who’s style you particularly like or would like to emulate to a point, bring them along, someone else’s viewpoint would be perfect for finding items you usually wouldn’t go for and would give you that confidence to try new things, plus they’d stop you making a bee-line for the kinds of clothes you are wanting to get away from when you enter a store!
Just have fun with it :)