I'm a big advocate of the read-through method to see if a piece is doable. If it's excruciatingly slow and hesitant and makes your head hurt or is still a big challenge, I would table the piece until a later date. There will probably be parts in a challenging piece that require some consideration of fingering and approach where you might stumble a bit, but if one can't follow and sound the main thread(s) of a piece in a read-through, then it might be a slog or uphill battle. Also a consideration is purely technical things that each piece focuses on with respects to one's history performing pieces. So consider what you have played and how it compares to the piece in question. Identify what is new to you in a piece, and how that newness compares to things you've worked on in the past. My general preference for new techniques is to find pieces that offer them in a more or less simple form, so I can approach a piece or technique stepwise rather than forcing myself to make a leap. I think one should be clear about what is technically and mentally necessary in each new piece, and if the piece is stressful to learn or feels as if one really needs to jump up in levels, to seek or ask for something that addresses those aspects but in a more approachable manner.
Rather than beating my head on a wall over a challenging piece that might take many months to learn, I like to memorize excerpts of what in the piece is challenging to me, understand what is mechanically necessary for sounding the passage as something that can be worked on and thought of independently of the piece, and to incorporate it in my warmup routine but sporadically (like, say, once a week or biweekly, even longer for very challenging, grail-like works). The spacing of the practice allows progress to be more visible, and it's very rewarding when months down the line, just testing the passage down the line, it magically comes into place. Over time you build up a large store of such passages. And if it doesn't come easily yet, you're just testing it out, so there isn't the added pressure or anxiety of you 'having' to get it down, because it's your main piece at the moment.
