Everyone has weaknesses. No matter how good an athlete may be, he or she probably has an Achilles heel. You can usually tell a player's worst weakness, because it's the area they least like to train. ( See How Katherine Reutter Turns Her Weakness into a Strength)
The good news is that eliminating limitations tends to lead athletes to the biggest gains in their training. Overcoming weaknesses is almost entirely an individual, mental toughness issue. It is based on the law of diminishing returns, one of the simplest sports psychology topics to understand, which states: the better you get, the harder it is to get even better, so more effort is needed to achieve small improvements. Conversely, you have the most to gain by curing your weaknesses, so that's where you should focus your training.
Bottom line: eliminating weaknesses needs to be a big focus of your training. Here are some steps to help.
If you're honest with yourself, you already know what it is. If you're still genuinely unsure, then pick the area of training you dislike the most. Even if it isn't your biggest weakness, you'll still benefit from overcoming this obstacle.
Before you can really attack your weakness, you need to look at it through different eyes. When I was a cyclist, I hated hills but loved time trials. I wasn't built for hills (I was too big and heavy). Then I heard Bradley Wiggins say that he just treats hills like time trials. This completely changed how I viewed them. A different mental approach helped me see how hills were less difficult than time trials. I just enjoyed them less. (See Failure is the Fuel of Judo Champion Kayla Harrison.)
Athletes love challenges, so make overcoming a weakness the same as taking on a big competitor. Attack it with gusto. Love the challenge, because, again, this is where you will get the most benefit. Move on from "Oh no, not hills today" to "Great! Hills today. This is where I will really make gains." Tell yourself this enough and you will start to believe it. Don't expect to learn to love it, just love the benefit you get from it.
You need to put more commitment behind training to fix your weaknesses. Hold nothing back and push yourself hard. You need total to be totally committed during every second of your training. You will make big gains, and you'll soon discover that your biggest weakness has become a strength. (See also Master The Five Pillars of Athletic Mental Toughness, Part 5: Resiliency.)