My Dominion/general card game policy is that I'm only allowed to get annoyed at a game if I can't find a single moment where I made a mistake. I can't actually do this all the time because sometimes both your opening buys fall on turn 5 and that's why you lose. However, I can almost always find some mistake that I've made. It doesn't even have to be big, and often isn't. Say, you didn't order the cards revealed by Cartographer correctly on one turn in a 20 turn game. There's your mistake, and if you had played that right, maybe it would have propagated to giving you a better chance in that game.
I've definitely gotten mad about losing streaks - when Iso was still around, over the course of a few days I dropped from level 37 to level 22. However, if you're serious about getting better at something, your goal isn't to win. Your goal is to get better. Ideally, you abstract your thinking away from the "win/lose" from a specific game, and think more in terms of the big picture. What mistakes did I make, how do I fix those, have I tried this strategy before, etc. Streaks in luck will normalize over time, so as long as you focus on improvement things should work out.