Should I keep wearing my retainer even though it hurts, or will I have to find an alternative solution? 1.5 years post-braces.

I see at least a few patients every week who report exactly what you're experiencing, so you're not alone! The great thing about retainers is that they can be passive, simply holding the tooth position, or they can be active, and actually move teeth.  When I have a patient who has been without a retainer for awhile and the teeth have moved slightly, the first thing we try is wearing the retainer FULL TIME, taking it out to eat, drink and brush only.  You can expect your teeth to be sore (similar to when your teeth were moving with braces!) and taking over the counter pain reliever as needed will help.  If you're persistent, the teeth should move back into their previous position fairly quickly (often within a few weeks) and the retainer will then fit comfortably.  However, in some patients the teeth have changed so much that this "jamming the retainer on" trick doesn't work and additional treatment is needed to straighten the teeth again.  If you're happy with the way your teeth look now, another option is to have a new retainer made to hold the teeth in their current position.  You then wear this new retainer as much as needed for it to fit comfortably, generally at least 1 night per week (or more if needed). 

Hello and thank you for your question. My recommendation would be to  see your dentist/orthodontist and consider doing a short term (like 6 months) braces again. This way your would be assured that everything will be done right. Depending on how much your teeth shifted, there is a small chance they might align back, but you are running a risk of putting to much pressure on them with your retainers and consequently increasing a chance of damaging the nerve inside.
Good luck to you!