Are a person's childhood circumstances a justification for committing sins, and is the statement that one should feel optimistic and have a good opinion of Allah after repentance correct?

Recalling the reasons for falling into sins, such as bad company, should be a matter of regret and future avoidance, not an objection to Allah's decree. There is no contradiction between fearing Allah and having good expectations of Him; both are required, and combining them is the correct approach. Praiseworthy fear is that which restrains one from sins, and praiseworthy hope is that which encourages worship. Exceeding these to the point of despair or feeling safe from Allah's devices is blameworthy.