What do you when you build up tolerance to zolpidem 12.5mg ER?

I also took the same sedative/hypnotic (same dose which is the maximum one can take every 24 hours) and after a couple of years it stopped helping.

I suggest you ask your Dr. about Seroquel (Quetiapine) off label use treats insomnia, which I also suffer from. Seroquel in low doses helps the treatment of insomnia/sleep disorders in most people as everyone reacts differently to medications.
It belongs to the drug class of 2nd generation atypical antipsychotics used to treat the symptoms of schizophrenia and bipolar disorder.
Don't let this scare you as this med is highly sedating, and for me it has done wonders, hopefully you'll give it a shot.