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Does this pain extend to the knees, into the shins or calves, and the feet?
Do you have trouble walking up stairs because your legs feel weak?
Does this pain seem to get better with sitting? How about leaning forwards?
Sitting in a soft chair -- does that help the pain?
Your symptoms really suggest an issue with lumbar stenosis:
and this could be from a disc herniation in the mid lumbar spine, as the L3 and L4 nerves would cause pain that radiates into the front of the thighs and cause difficulty like you are describing.
Given the severe pain I would discuss further with your GP to see if imaging of your lower back is needed. Certainly pain that persists despite treatments like physiotherapy, ibuprofen would warrant MRI imaging of the lumbar spine.