Hi, I'd suggest trying different maps, AI, and/or AiX cheat modifiers. You could also try fighting multiple AI at once (e.g. a 1v2 or 1v3 game), or playing at a higher game-speed (e.g. the AI will be much harder at +4 speed since its apm is unaffected).
You mention you're turtling - M28 and M27 try not to attack a turtle directly unless confident of winning (since suiciding units into a turtler is usually a bad move), but some AI will be more aggressive. DilliDalli is one such AI, but I'm not sure if it's still working following a patch some time ago. You could also try RNGAI (it's not suicidal in the way DilliDalli is, but my impression is it's generally more aggressive than M27/M28 when it comes to turtlers).
Some maps like astro craters will naturally incentivise turtling play, but if you play on large open maps then if you turtle you'll be left with such little map control that you'll be crushed.