Believe it or not, you're not the first bride to feel this way. It's a lot of pressure trying to balance everyone's ideas without hurting any feelings. And let me say, it's not even possible because if you do it all "their way", your own feelings may be hurt in the end. Not how the princess of the day should feel.
The solution, listen and speak with your heart. Rarely are those bossy family members really trying to make it about themselves (even if it feels like it). The goal is most always to make it as perfect as possible for you.
Often you can head this issue off early by saying "this is the style we want to achieve for our wedding" and then invite them to offer ideas and suggestions "within your style" that they could help with. However, be very clear that you are having this same conversation with both families and maybe even your friends so you need ideas, not actions. This way you've established that you do want and appreciate their help while still maintaining that you are in charge and other people's ideas are important too.
Presto, you've not offended anyone! Maybe even more importantly, they realize immediately that their idea may or may not be used rather that having to deal with hurt feelings later because they've "surprised" you by putting a deposit on a caterer you don't want or bought supplies to make centerpieces that don't match your theme.
Worried this makes you sound selfish? It doesn't at all. In fact, your parents will likely be very proud of your sense of responsibility to ponder your options before putting your plans in place.
Bottom line: Communication is key!