I've copy and pasted my response from a similar question, but if you have any other questions feel free to ask.

I would be hesitant to say that the Soviet defeat in Afghanistan meaningfully contributed to the fall of the USSR; rather, it was only the latest outcome in a process of Soviet decline that was already underway. The Soviet economy was lagging and the international influence of communism had diminished greatly by the early 1980s. Gorbachev was well aware that Soviet communism as it had been practiced for the last four decades was no longer tenable. In response, he carried out a series of liberal reforms under the programs of glasnost (openness) and perestroika (rebuilding). For Gorbachev, the Afghanistan War threatened his ability to effectively carry out his reforms. Domestically, Soviet citizens believed that their government was wasting resources on unnecessary foreign interventions while they suffered through economic hardships at home. Ironically, Gorbachev's glasnost reforms meant greater freedom to critique Soviet policy, thereby increasing the pressure on Gorbachev to withdraw. Even without the domestic criticism, Gorbachev would have likely still withdrawn as the war exacerbated the problems of an already weak economy hampered by declining prices in key Soviet export commodities. Moreover, most historians agree that, if the Soviet Union had continued the war, it could have withdrawn in a much more favorable position within a couple of years. Gorbachev, however, wanted to get out much quicker in order to ensure that his social and economic reforms could be instituted effectively. In other words, it is probably most accurate to say that the Soviet-Afghan War accelerated, but certainly didn't in any significant way cause the collapse of the Soviet Union.