To stop a rushdown you have to use your footsie game to limit your opponent’s options and therefore stop them from getting into a position where they can gain an advantage (e.g. a knockdown) effectively rush you down. Of course at times the best answer to a great offense is to have one of your own, and therefore it would definitely be good to develop your own rushdown game. Of course this depends on which character you are using and if rushing down is one of his/her strengths.
When being pressured, i.e. rushed down, BLOCK. Don’t mash out and don’t be too hasty since that’s usually what your opponent wants you to do. Be aware of your opponent’s block strings and habits and you will be well on your way to diffusing rushdowns. When you see an opportunity to effectively reversal or have a high priority counter to a move in your opponent’s pressure game, make sure you take advantage in a timely manner.
Developing a solid defense takes time and you are going to get rushed down, beat up, perfected, and taunted in the process, but stay calm and motivated and eventually you will find that you are doing most of the rushing down and/or perfecting, taunting, etc. lol