Why was Gorbachev unpopular in the U.S.S.R.?

Gorbachev's lack of domestic popularity  was a result of seeking to liberalize a nation that had been steeped in centralized rule for decades.  Given that there was no infrastructure within which to support the liberal changes he advocated, internal chaos within the Soviet Union resulted.  Essentially, "freedom" and "openness" brought with it a lack of control and new problems that were unforeseen. The same spirit of life and vitality that the West praised caused Russian citizens to feel threatened and challenged.  The encouragement of a decreasing military presence in Europe resulted in formerly controlled republics declaring independence and breaking away.  At the same time, Gorbachev's encouragement for labor reform led to strikes and disagreements with labor unions, as seen in Soviet coal miners' strike of 1989.  

The reality of an underdeveloped and impoverished Russian nation that had been shrouded in Communist propaganda for so long became evident to the world and to Russian citizens.  Gorbachev had broken through this wall of silence. When confronted with such challenges, they proved to be too difficult for him to control.  His lack of popularity stemmed from his inability to address the realistic conditions of freedom that he himself encouraged.

Why did Gorbachev face opposition within the Soviet Union?

Mikhail Gorbachev met with opposition in the Soviet Union largely because he wanted to implement major changes.  People who want to change systems in radical ways are often met with opposition.  He also met with opposition because his reforms did not bring about any sort of immediate prosperity.

When Mikhail Gorbachev came to power in 1985, he did not want to bring down the communist system.  However, he did want to reform it in very important ways.  He wanted to open the system up both economically (perestroika) and in terms of political rights (glasnost).  At that point, the communist system had been in place for almost 70 years.  Anyone who wanted to change it in radical ways was bound to encounter some degree of opposition.

In addition, Gorbachev’s reforms did not really work all that well in the short term.  The new political openness led to political conflicts.  There was much more discord within the country with (among other things) some parts of the country wanting to break away and become independent.  The new economic system led to widespread problems as well.

Thus, Gorbachev encountered opposition both because his reforms were radical and because they did not achieve immediate success.