I can understand the need to save information – but always wonder how email has become such a hoarding problem. If there is correspondence that you need to hang onto, would it be possible for you to use the PC to hold the bulk for you? Those attachments could be moved over to file folders, leaving your pst file with spare room. Not the answer you are probably looking for, but perhaps something to think about to prevent issues down the road.
There is a possibility the pst is corrupt at some level. Run scanpst and try again.
To fix pst file issues you can try this PST Converter Software
and easily fix the issues with corrupt .pst email backup files.