Internal dissent and defections also pose a serious problem for the junta.
Reuters spoke to two former policemen who secretly defected and aided the opposition by leaking intelligence.
Yan, who is now 24, fled Myanmar in April. Reuters is not disclosing his whereabouts for security reasons.
[Yan / Former police officer]
"I want a future for Myanmar where the military cannot stage coups again and again. I want a military that will listen to the President of the country."
Yan joined the police force in late 2020, but decided to defect after he witnessed the arbitrary arrest and beating of protesters during a brutal military crackdown.
"People hated us," Yan told Reuters, adding that he "didn't feel good."
Yan is living in exile with his elder brother, Ye, who also defected from the police force.
They started a business making and selling fabric printed with leaves from their home country. Some of the proceeds are sent back home to support the resistance.
In October, Myanmar's rebel coalition launched a major offensive against the ruling junta, capturing around 150 military outposts, five towns and four border gates within a month.
This video released by an opposition armed group in November purportedly shows soldiers surrendering to the insurgents.
One of several opposition groups told Reuters they have been receiving intelligence from spies within the security forces.
It included information about the transportation of military supplies, which helped them plan ambushes. They also received tip-offs about impending attacks or air strikes.
Think tank United States Institute for Peace estimates as many as 8,000 people have fled the security forces.
The brothers also said the junta was struggling to recruit people while police were poorly equipped and trained.
[Yan / Former police officer]
"They surrender fast because there is no proper discipline at the stations. They are always drinking alcohol and there aren't enough police members. In actual fighting, they cannot fight. They get scared. So, they surrender easily."
The junta didn't respond to Reuters' request for comment, but has acknowledged some loss of control.
The military, known as the Tatmadaw, has ruled Myanmar for five of the past six decades.
