Its quite common to start a painting and not finish it. There are many reasons. It may not be working out as you imagined it; you may lose interest in the subject; you may not have time or other projects are more pressing.
I don't toss a lot of paintings, but the ones that I do put aside are usually returned to and reworked at a later date. And that date can be years later. I think I must hold the record for not returning to a portrait for over 20 years! Unless I plan to completely rehaul the painting, I don't gesso over it, but simply oil out (if using oil paints) and make my changes.