I strongly suggest you select just one race and stick to it. Playing the other races is good to get an understanding of how the other races work and what their limitations are but at the end of the day you will improve fastest by trying to improve just one of the races.
If you're genuinely stuck then there are several options available to you that you may be unaware of.
1. Coaching: Yes this costs money, but these coaches have been where you are and know how to get where they are. Like personal training at a gym coaching will speed your results allowing you to obtain your starcraft goals faster.
2. The replay feedback thread! (click here) This thread is frequented by many helpful people willing to look over your replays (especially replays you're stuck on) with the goal of helping you improve. Often times these helpful people point out mistakes that you didn't even know you were making.
3. Watch streams: Watching professionals play (especially when you try and analyse their play) will help give you an insight into why decisions were made and concurrently improving your decision making skills.
4. Join a Clan: I cannot emphasise this one enough, a clan is full of helpful people often who share goals similar to yours. Some are in a clan just to fool around of course but others are there who want practice against people of your skill level. Which leads me to point number 5.
5. Practice partners: These are people who wish to improve much like yourself. They will look over your replays and you will look over theirs. You will help each other get better at the game. Clans are an excellent place to find practice partners as well as this thread. EDIT: You can use the quick find feature found in the features menu above or click here.
If you're genuinely stuck chances are you're doing stuff wrong that you aren't even aware of so hopefully one of the 5 tips above helps you out.
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