Stretching is a challenge. Every time you are trying to get your muscles and tendons to stretch a little bit more, go a little bit further. This leads to suppleness that helps in the degrees of biomechanical freedom available to you as you ask your body to move. The price you pay, each time is pain. Stretching always hurts, no matter how you do it. There are two ways to get better at this and you will need both. First, the incremental approach. You can get down to doing perfect box splits but it will take time and patience. You simply cannot rush it. So prepare yourself mentally for that and be disciplined enough to actually do it. Second, the habituation approach. In order to get used to the pain of stretching and also see results in the stretching itself you need to get your body accustomed to doing it every day.
If you follow this approach you will find that you do become more flexible, your muscles more supple. Flexibility also helps you become faster as your muscles no longer fight against their own tightness every time you execute a move, which leads to a corresponding increase in power.