Puzzled with the result of LM317 constant current
If your LED lamp assembly is 10W at 900mA it needs about 11.1V to operate. The LM317 current source needs 1.25V in the resistor and a further 3V headroom to operate, put it all together and your circuit needs a supply of at-least 15.35V to behave as you intend.

At that voltage and current the LM317 will be burning over 3.8W so  you'll be wanting a heatsink on the LM317...  You're probably better off with a switched-mode led driver, you can get an assembled PCB on Ebay etc for a few bucks.