You have great blessings for having the ability to maintain the Shurangama Mantra.
What you are experiencing is called karmic obstructions.
The Vietnamese have a saying: “The ten thousand undertakings are very hard to kick-start.” It takes time and patience before you can get your momentum.
Just patiently endure the discomfort. Keep at it and it won’t be long before the discomfort abates and the bliss increases. It will be worth it if you resolve to endure the difficult start because you will gain inconceivable benefits with the practice.