Having spent two years on something is a reason to learn from it, not a reason to spend two more.

Time already spent cannot be recovered by spending more of it. The pull to continue comes from wanting the past investment to have meant something, which is understandable and also the wrong question. The useful question is whether the next two years are worth what they will cost. Whatever you learned stays with you either way.

Ask yourself whether you would start this today, knowing what you know now. If the answer is no, that deserves to be taken seriously rather than argued with. Walking away does not delete what you gained.