After you’ve achieved your goal, take a (short) moment to celebrate. Be joyful in the success that you’ve had. But don’t stop moving! You’ve just spent all of this time and energy (and potentially money) building the momentum to achieve this goal! Don’t stop doing what you’ve been doing. Set your sights on the next goal!
If you get complacent, you’ll lose all of the momentum that you have in your favor. You will have to restart, and reaching that next goal will become more difficult because of it.
Think of rowing a boat. It takes less effort to keep the boat moving than it does to get it started from a dead stop. The hardest part is the beginning. Don’t shoot yourself in the foot by stopping and starting over again.