Am I entitled to think good of Allah despite the existence of some major sins with which I have been afflicted and my attempts to abandon them?

There is no issue with having good thoughts about Allah while committing sins for someone who strives against their self and tries to abandon those sins. Indeed, the sinner's belief that Allah will forgive and pardon them if they repent and seek forgiveness is a form of having good thoughts about Allah. The issue arises, however, with the disobedient person who persists in their sin and disregards the limits set by their Lord, yet still hopes for pardon.

A believer must combine hope and fear. Neither should the aspect of hope become so overwhelming that one feels secure from Allah's plan, nor should the aspect of fear become so overwhelming that one despairs of Allah's mercy. Good thoughts about Allah should be accompanied by avoiding sins, undertaking actions that bring good, and abandoning actions that lead to evil.