Hi, I'm Robert and I'll be helping you with your computer question.
If the size is a problem you can try creating new PST files and loading them all into outlook and moving emails to the new ones so you have 5 PST files of 3gb instead of 1 of 15
You might also try running the scanpst program that comes with outlook to compact the file and check for errors
Before that you should
Make sure the deleted folder is empty
And run the compaction routine under tools maintenance