HID lights confusion

[BOUNCERCHICC]

I'm new to this sight, so please help me. I have a Nissan Altima 3.5 SL 2006 (purchased in 2006). I have HID lights, but they seem to have become dimmer. Why? especially if I drive at night only 3x a week. If I need to get new lights, do I need to buy the HID Conversion Kit, or can I just get the bulbs. I would like the 6000k Diamond Blue ones. Also, can I do it myself? Help explain this stuff... Damsel in Distress