You have to put things in perspective. Were you better later on in your training than you were when you first started? Were you better in comparison to who? All of that makes a difference and is equally important as being the best.
Sucking at something means a person is just as horrible as he/she was when they first started. They haven't improved in any shape or form after all of that training. If that is happening, then a person can truly say that they suck at it. It doesn't mean that you have to stop training because it may be something you enjoy doing, but just embrace that you suck and keep going. It's like people who suck at singing but still sing. My wife is a horrible singer and it hurts my ears but I know it makes her happy so, I take the hit lol.
Now if someone sucks and that makes them unhappy, then that's when it's time to call it quits. In terms of sports, sometimes it's a wrong place and time situation. My first year in high school track was nothing but me losing. There was a lot of of competition from the senior class. By the time I was a senior all of my tough competition were long out of high school. I made it to City but not State. I won about 97% of my races. Part of my success was because I trained hard. The bigger part was because those who were better graduated. lol