This is the one question that will get you to 99% of your decisions when it comes to letting go or keeping pieces in your closet. It's simple and ridiculously effective:
If something serves you, it should stick around for the long haul. If you have a dress that you feel confident in every time you wear it, by all means, keep it and buy five more in the same shape or print or both. On the other hand, if you have pieces in your closet that don't fit well, don't feel good on, or worse, you feel very 'meh' about after trying them on, you should ask yourself why you're still holding on?
Why hold onto something in your closet that makes you feel anything less than good when you wear it?
Why hold on to anything in your life that makes you feel anything less than good while you're at it?