Are previous prayers considered invalid and void due to a defect in creed (such as shirk and disbelief), and must they be made up after repentance, or is repentance sufficient, and one only needs to start with current prayers?

You are afflicted with waswasah (whispers/obsessive thoughts), and you must be cautious of it and not give in to it, especially regarding matters of disbelief (kufr). Your love for other than Allah, when it equals your love for Him, is pure waswasah. Not every instance of pessimism or discontent leads one out of the fold of Islam. You must discard these obsessive thoughts and not pay attention to them. As for the prayers in which you omitted some conditions or pillars, the obligation to repeat them is a matter of scholarly disagreement. There is no harm for you in following the opinion of those who rule that there is no obligation to make them up, as long as you are afflicted with waswasah.