Both were significant mistakes, IMO, although turn 8 one is much bigger.
Minion is one of these powerful cards which, when you see it on the board, must immediately ask yourself: Can I build a strategy around it or not?. And here the answer in obvious yes. So, open with Steward, buy one Junk Dealer and then start massing Minions. Since there are no villages, your goal should be to play as many Minions as possible (compounded by non-terminal JD trashing) and one terminal. Which terminal it is - I'm less sure: maybe Butcher and maybe Merchant's Guild.
In light of such approach, buying T4 Gold is a waste of time. You don't need Gold - your money will come from Minions. And turn 8 is, as noted, a major mistake: you replaced one junk card (Estate) with three (Farmland and 2 Ruins), effectively undoing your important trashing from previous turns.
As to whether your game was lost before T8 - possibly it was, as a combinations of unlucky Steward missing the Estates and T4 Gold (trashing here is optimal, and if you had to buy something - buy JD rather than Gold). Your opponent certainly has a significant advantage, and you'd have to rely on him erring or having bad shuffle luck in order to win this game.