It's almost a year I'm playing Viola and my biggest problem is with my left hand's thumb. I cannot put it in a relaxed situation. Actually it will be contracted during the practice. My teacher told me to think about it and you should fix it mentally. But I'm really frustrated and I can't fix it.
Practice a relaxed while controlled thumb stance throughout the day. Your right arm is a makeshift violin neck when you are idling. Play and shift positions on it habitually. Eat with your left hand using chopsticks for a month (the chopsticks may be even more relevant for the bowing hand but the problems from badly controlled bowing technique are more subtle) or however long it takes you to use them confidently and in a relaxed manner.
Playing the instrument is when you put all elements together. Which is important. But that does not mean that training of single components should depend on it.
Also "it's almost a year" you've been playing viola. That's nothing. Violin and viola are quite unergonomic instruments. Until they become a reasonably natural extension of your body, you can easily spend half a decade. And you never really stop working on it.