Strange, that you see previous version as a trial. You own your previous version and there should be no reason why it was deactivated. Maybe try to call Adobe info line if they see your serials as deactivated and try to activate your CS5 again. Did you upgraded to box/licence of CS6 or to Creative Cloud? On our Mac's we quite often have CS4, 5, 5.5 and now 6 alongside and all versions are working correctly.
But we have issues with PS CS6 too, usually after few hours of work with PS when saving .psd to network server.
In the past, I have had PS version 4 (needed that for one particular way that it handled Spot Color, for one client), PS version 7.0.1 and PS CS (note: on version that required Activation), and never had any issues. However, only PS CS used the Activation process.
I second the suggestion to contact Adobe Customer Support (not Technical Support), to sort out that Activation/licensing issue with PS CS 5.5.
In decades of using Adobe products, I only recall two issues with other Adobe programs. Going back a bit, if one had Premiere Pro CS 3 installed, then installed PrPro CS 4, and subsequently uninstalled PrPro CS 3, that uninstall took some files with it, that PrPro CS 4 needed. The fix was to do a Repair Install of CS 4. Then, if one had CS 2 (full Production Studio in my case), and installed Premiere Elements 4, every program in the Production Studio (now called Production Premium), broke. Again, that fix was a Repair Install for the Production Studio programs.
Currently on my laptop, I have several versions of PS, PrPro and PrElements installed, and all working side-by-side, with no issues.
Good luck, and hope that you can get PS 5.5 to Activate and work for you.