P0137 Fault Code - 1997 Blazer 4x4 4.3L Vortec Engine Diagnosis

First time posting here, hoping for some expert advice. I recently received a P0140 diagnostic code, so I replaced the oxygen sensor located before the catalytic converter. After that, the vehicle started throwing a P0137 code. The truck runs roughly during initial startup and lacks power—especially on first ignition—and only improves after warming up for about 20 minutes, but only when the engine has been shut down. I performed a major engine rebuild approximately two months ago, involving significant labor and parts costs. Despite having invested heavily in this vehicle, it's now become more of a mental challenge than a mechanical one—I'm determined not to let it defeat me. This situation feels similar to dead batteries in a TV remote: you keep pressing buttons instead of turning off the device and trying again. I'm looking for accurate troubleshooting steps specifically related to P0137 on a 1997 Blazer 4x4 with a 4.3L Vortec engine, including possible causes like faulty oxygen sensor, wiring issues, or ECU misreads after the rebuild.