Hi Bucks, boy do I understand what you're going through. I also remember a time when my reflexes were faster. Here are a few ideas that will hopefully help.
1. When the ball is within shooting range, always be in ready position. That is to say, on the balls of the feet, knees bent, chest slightly forward, hands in ready position (either "piano" position or palms open). This will make it easier to react to a shot since in theory you'll always be ready, even when you're moving.
2. Set your feet when you see your opponent about to strike the ball. Though a good shuffle can help you make the save even when moving, your best chance at a save will be if you're in set position. If you are still shuffling when the shot is on its way, it will be harder to react. So when you see the opponent about to strike, get in ready position (even if you're out of position in regards to the goal and the ball). Once the shot is coming, be ready to react. In this moment I try to imagine myself as a coiled spring, and once I get a read on the ball I explode to it. Shuffling will hurt your chances because it limits your downward, upward, and lateral mobility.
3. Prevent shots by organizing your defense before pressure comes. Prevention is the best way to stop goals. Make sure you know the names of your teammates, and communicate with them often. Don't give them technical direction (what to do or how to play), but give them tactical information (marking up, run coming back door, drop back, etc.). The better your organize your defense the fewer chances will get through.
4. Lastly, work on strengthening your quick twitch muscles. Your slow-twitch muscles give you power, your quick twitch give you quickness. You can find countless videos on YouTube about increasing agility, but in general any movement that has quick repetitions and is explosive in nature (agility ladder, box jumps) will help your muscles speed up.
Hope that helps, good luck!
Brian, TIP Goalkeeping