A Contract Line Mechanic reports he failed to reinstall an axle nut washer (spacer) after replacing the #1 tire at the left main landing gear (MLG) on a CRJ-200 aircraft. Tire Shop noticed the washer stuck to the wheel hub of the removed tire.

Narrative

I work for XXX Line Maintenance Contractor at ZZZ airport. While I worked on a CRJ-200 aircraft for Air Carrier X the two events stemmed from a #1 main landing gear (MLG) tire change I performed. I received the call from [Air Carrier X] Maintenance Control at around XA:00pm local [time]; to inspect damage of #1 MLG tire. After receiving the limits via fax; I inspected the tire and found it to be beyond limits. Maintenance Control advised a tire was going to be sent from another station on Air Carrier Y's aircraft along with the paperwork via fax.The tire arrived at XC:00 local. I received the call from Maintenance Control to proceed with the tire change. After skimming through the paperwork I proceeded with the tire change. Here is when multiple factors played into the mistakes I performed.First; not deflating the tire fully; which later was put on the aircraft by a co-worker. Second; failing to install a spacer [washer] which was not removed from the unserviceable tire. I clearly rushed through the Maintenance manual due to complacency and to get the plane out on time after Maintenance Control stated the pilots had an hour before they timed out. At the time I thought a tire is a tire; they're all the same. I looked for the key points like torques and safety wiring which ultimately led to my mistakes. I know what I did was wrong and I definitely learned from it. I will never again jeopardize my licenses and career like this again.Other factors that played into this were the time of day. It was dark and led me to miss that spacer. I still had work on my overnight aircraft and [did] not have the proper tool in hand to deflate the tire; which led me to only partially deflating the tire. Due to the events; I have been put on six months probation; have to re-complete human factors training and read memo from Air Carrier X; which informs about the spacer on this particular tire and how easy it is to miss.