Just want to underscore a few points that have come up so far.... As Ranjit said, avoid mislearning is very important, taking it slow can help with this.
I might add that we often say that if you can't play anything in a new piece HT, say a few measures, or the opening section (which is often the easiest), then that's a good sign the piece may be too difficult for your current level. But strategic use of HS practice can also be very good, esp for more beginning players.
Lastly, fingerings. There's no shame in writing them in, and it can often really help speed up learning because you have a greater chance of using the same fingerings each time you play/practice that passage, which really helps with muscle memory.
Sometimes, with more difficult pieces, one of the first things I do is go through an write in fingerings (usually section by section). This feels inefficient, because just choosing and writing them in is time consuming, but in the long run it's actually more efficient because I cement those fingerings faster.