As the wedding day approaches I suddenly feel doubt and fear. Is this normal, or a sign I should withdraw?

Nerves and doubt before a wedding are common; what needs distinguishing is fear of a big change versus well-founded doubts about the future spouse's character or safety.
Allah teaches trusting in Him after making a firm decision (Al ʿImran 3:159), and the Prophet ﷺ taught the istikharah prayer for important matters (Sahih al-Bukhari 1166). Helpful questions: are these doubts about yourself (responsibility, freedom, money), or about specific things in your fiancé(e) such as not praying, a bad temper, lying, excessive control or violence? The first usually eases with preparation and discussion; the second must be taken seriously even if wedding preparations are far along. Talk to someone you trust or a counsellor. Breaking off an engagement is lighter than a marriage that puts you in danger. See the question on istikharah and ending an engagement.