I play Terran and I find that I can demolish very hard level Zerg quite easily, but I struggle against very hard Protoss. I cannot beat insane at all, because he macros me out of the game too easily. I haven't played my placement matches. I am a newbie and I want to learn the game and all 3 races before I play human opponents because that's just the way I do things, plus I want to get into the right league as I know that moving between leagues is an issue for some. So, any ideas about what league I fit in at the moment, roughly? And how do platinum and diamond players fare against very hard and insane AIs? And does anyone else feel that Zerg AI is the weakest?
I recommend playing human players a lot. Playing the AI will only help you with the basics of knowing which units do what. Humans and AI play very differently, so you're not doing yourself any favors playing the AI first. If you are avoiding league matches because you are worried about your initial league placement, you can try joining one of the custom games. Specifically, the ones that say "blizz" next to them, like "Lost Temple blizz" because then you can play against a human on that map without it counting towards your record. That being said, I highly encourage you to just get past those awful placement matches and start playing against humans of your skill level. The best way to get better at humans is to play lots and lots of humans. Some will zerg rush you, some will turtle, some will proxy-pylon you, and some will just out-macro you. But each time you see a human do such a strat, your brain will start to pick up on how to watch out for it, and you'll claw your way up the ladder.