You can watch your tachometer to see if the engine is slowing down. Try putting on the brakes when it is running properly and see what happens.

If it doesn't stop in first gear my guess is the clutch is slipping. Do you feel hard pressure on the clutch pedal as soon as you push it? Or does it move some and then have resistance? The free travel in the clutch may need to be adjusted or the clutch may need to be replaced. WB.