Did you make a big mistake? Yes.
OSX uses a file format incompatible with Windows and creating a Windows partition would not change that, those partitions would still not be accessible. Unfortunately your data may be lost.
My suggestion to see if you can access the data is not to reformat yet. I would download and install the 10 day trial of
HFS for Windows and attempt to mount the OSX partitions on a Windows machine and copy the data from them to another drive. Then reformat the drive.
If you want to share a drive between Mac and Windows, format in FAT or if you have newer Mac and Windows version you could also use exFAT. I would probably do that format on the Mac.