The problem was that armies got a new emperor bonus, basically, and therefore had a constant incentive to proclaim their general emperor, and this happened a lot. Gaul and Iberia fell under the sway of a breakaway emperor, and the Palmyrene Empire (effectively led by Zenobia and her husband, then Zenobia when her husband died) arose in the East. Zenobia also managed to take over Egypt, a key province for Rome.
Gallienus was succeeded by the Gothic Claudius, a fantastically talented but short-lived emperor who probably had a hand in his predecessor’s untimely death. He himself died after just a year in the job (of plague rather than being murdered) and was succeeded (after some quibbling from his brother) by Aurelian.
The empire had been fracturing and effectively rudderless for some time. This had meant that soldiers that should have been guarding the Roman borders were being pulled away to fight their own side and reunite the empire, making it spring time for the barbarians wanting to raid the empire. At the same time, Zenobia and her husband (and son, later) were left alone and their authority was sort-of-recognised. The Palmyrene Empire included much of Turkey, Egypt, and modern day Lebanon, Syria and bits of Iraq.
Unfortunately for Zenobia and her young son, Aurelian was just as skilled as the Gothic Claudius and once he’d finished slaughtering the Goths, Vandals, Juthungi, Sarmatians and Carpi (and beating the breakaway Gallic Empire into submission) he turned his attentions east.
Initially Zenobia ventured to battle with the emperor, but her forces were crushed, and she was captured and taken to Rome. The queen appeared in golden chains during Aurelian’s victory parade. It’s unclear whether she got her head lopped off, died of disease or lived (briefly) in reasonable comfort. (This era lacked the excellent histories of earlier times due to the rampant inflation, frequent civil war and common killing of emperors, alas).
And so the short-lived Palmyrene Empire was strangled almost at birth by Aurelian. Had the Gallic Empire enjoyed a similarly talented emperor or Aurelian himself been murdered a year or two earlier Zenobia might have remained a ruler and the Palmyrene Empire might have become a long-term buffer state between the Roman Empire and the Persians.