When your work doesn't pay off

Hi guys! I need help, if you can please assist me. I used to swim competitively when i was younger up until i was 17, and back then i was improving a lot, very fast. I am extremely frustrated, as all i want to do is to become a faster Masters swimmer, and be the best that i can be. I'm still young so I want to take advantage of that, i came back to the pool about two years ago after being off for about 8. I'm doing cross training with Running intervals 2x/week for 45 mins, doing a weight routine designed for swimming 2X/week, and swimming with my team 5-6X/week. I've been doing this for about a year now, and my times have not changed at all, or have changed very little. I am investing a lot, I even have a nutritionist to help me with a good diet to support my workouts, and nothing. I was told that What's worse, I see people who seldom go to practice, or don't train all that much and they are quite fast. I feel like all the work that I am putting in is not paying off at all as it should be. Any help, advise or anything would be hugely appreciated.