Analog stick won't respond when pressing up

I've had my Wii U for 2 and a half years and it's run perfect. I play a lot of Splatoon so movement sensitivity is essential when playing. Only recently my left analog stick doesn't make you move quickly when pushing it up fully. In most games, lightly pressing the analog up makes you walk, that is exactly what happens when I move my analog all the way up for a few seconds until you jiggle the stick a bit. It only responds like this when pushing forward. You either don't move at all or take the slowest steps. It makes playing a game like Splatoon horrible. I know it's not the game because it does the same thing in several other games, just it doesn't bother me as much in those other games as the movement isn't part of competitive play. I was assuming it's sensitivity is worn out due to the amount of time I spend playing Splatoon but then again no other controller i've ever had in my life has had this issue from over use. The warranty has run out on my system and they will repair it for 115 dollars. I really don't want to spend that much if it's a simple solution maybe. I've never dropped it, no physical damage, tried re-syncing the sticks and still nothing. It's very frustrating. Hopefully someone here can help me figure out what's wrong and if it's worth trying to fix myself or send it for repair for 115 dollars.