Outlook PST error

I keep my old emails in the pst file.., I have around 20 pst files to keep achieve my old emails. Each month I move emails from different senders to different pst file .. but I can only open 15 personal folder at one time.. I notice when i open the number 16 personal folder, there will be an error telling me there is an error opening the pst file and I have to close down the outook and open again.. i am using outlook 2003.. it is the constraint of outlook 2003??
please advise