I took metformin for two years for PCOS. The first year was hell as it was a never ending battle with my stomach.
The second year, they put me on the extended release pills and that was much better. I was able to tolerate it at lower doses, but then my period stopped again, so they started pushing up the dose as high as they could, which wreaked havoc on my stomach again. I eventually had to go off of it all together.
However, I have been told by many doctors that most patients do much better on the extended release, when it comes to upset stomach and nausea. Have you tried this? I also have IBS, so I think that's why I had so much trouble. I also think eating sugar has some kind of weird reaction with the drug, as I found my symptoms were worse, if I had something too sweet. When I stayed away from refined sugars (as I should anyway) I found I tolerated the drug more.
Anyway, if you haven't already I would ask your doctor about Metformin ER. It is much softer on the digestive system.
PS - The good news is, most people, including myself, lose weight on the drug. I lost 8 pounds in one year! So, that was a bonus at a time, when the drug was not so good to me.

TTC for 3.5 years.
Male infertility factor; female history of PCOS.
1st cycle failed when all 8 eggs arrested before reaching embryo stage.
On 2nd round of IVF w/ICSI - 2 embryos transferred.