do you play mouse on the left side or right? this question is no ambitious people who can both as good. 
i recently switch into left side and notice that my micro was getting better and more accurate…
if you try to play on the right side and youre lefty… eventually you will realize youre not using ur natural hand because brain needs time to mirror what left hands tries to do to do and send the mirrored signal for the right hand…
…no?
I am a right handed, but the statistic show, that most of the left handed use the mouse with the right hand.
I can tell you that once I moved my binds on my keyboards by 1 to the left (production from 7 to 6, nexus from 6 to 5 etc.). It was horrible, I pressed the wrong keys all the time for few month.
Same happened when I was training MMA and I had to learn how to fight in left sided position too. I had the same problem, that you describe here.
So my advice is: it takes A LOT of time to change or train the muscle memory (especially if you never used the mouse with right hand) and I think that your current problem is that your need more time to adapt.
Just an anecdote, but I fit into that category.
Every computer I ever had used a right-handed mouse, and the presence of most controls and hot-keys on the left-side of the keyboard meant that it was usually better to stick with that setup. I “can” use a mouse with my left-hand, but because I’ve been using the standard setup for so long I barely get any benefit from it.
I have heard this story before and it wouldnt matter unless you go for the very top… Left hander can perform uhm almost as good as uh… mediocre right handed player (using mouse on his righty)… but if left hander wants to be a jimi hendrix of scII he needs eventually switch to use his main hand where the reflexes are most natural. 
I already perform better on my left playing scII. To compare it with leagues I would say I can be solo platinum on my left and bestly gold 1 with my right.
I just want to spread this message to fellow left handers that we can – uh – manouver in this right handers world and we wouldnt have to always assume we perform worse than right handed peers.
Im left handed and i hold my mice right hand no need for deep philosophical questions about it don’t be such a Terran…
Maybe the left handed using the mouse with the right hand compensate by being more effective at using groups and keyboard shortcuts since that use their left hand?
Actually, since “the left side” is much more complex to deal with than a mouse, maybe by default or by statistics left handed would be better?
I’m left handed and have the most in my left hand i move much faster and can micro so much easier
yea same bro same… i play like 15 years mouse on right but 3 years now on left… i had to start all over but was pretty quick to understand thats its still the same game… i love it now more ! =)