Dominion is streaky, really, if you have two intelligent people playing who understand the board, it's a 40-40 split, with the "skill" argument at around 20%...I have beaten players with a lot more games under their belt, probably much better players than me, but I beat them because I had a decent strategy, I got decent draws and sneaked a victory. So, Dominion can get streaky, I wouldn't sweat a 10-game slide, I'd sweat a 50 game slide though.
As for Masquerade, I understand both opponents are trashing, but Masquerade games are slow and while I am sure its a great opening, I have trouble winning when I open that way. Meh, Masquerade, I'll keep tinkering with it, but at this stage, I am not utterly convinced its the strongest opening. It just seems like a slow trashing mechanism with slight card gain (people hate when I use that word in that context, card positive I guess).
I just won a Minion game on boardgamearena. First time pulling that trick off. I screwed it up a few times and messed up my deck with pawns. I think the key to Minion chains keeping the deck balanced. I thought the Pawns would help, and at first they did, but as the deck got wider with land acquisition they were a pain in the ass. Live and learn I guess.
But for me, "decline" often means you are experimenting, which is a good thing. Simulators only reveal so much (and they can't make the subtle adjustments beyond an algorithm that sometimes you need to do in REACTION to what your opponent does). And I don't think solitaire is a full practice either. It gets you an idea on tempo/balance of a deck, but to compete you have to face an adversary who is reacting to your construction. Which means, the only way to learn is to get in there and try stuff and when you do that, you lose.
Iso is brutal because, I am convinced some players create new accounts to get a "clean slate". I've played level 0 players, who clearly not only know the game, they know it well, play fast and are razor sharp. This tells me they are likely someone who wants to rebuild their rating from scratch, possibly even challenging much higher level players to do it. On top of that, its EVERY expansion, which I can't play test at home.
So its a fiery, competitive arena and as you broaden your skills, your going to go out of your comfort zone and lose. That's currently where I'm at. Is Steward a good buy? Is Lookout? Is two Ambassadors better than one? Do you reach for the Province on an early draw of 8, or reach for another Gold? When is Harem a good buy? All those decisions are made for you in BM+ and BM++, but it becomes a vaguer, muddier world once you abandon BM in order to grow.