I am strict with my children because I love them and want them to succeed, but lately they seem afraid of me instead of close to me. Have I mistaken harshness for love?

What you are describing is a common thing in good homes, and it is not proof that you have failed as a father. Strictness and love are not opposites. But firmness standing alone, with no warmth beside it, can feel to a child like a wall rather than an arm around the shoulder. Children read our faces long before they weigh our reasons.

The Prophet, peace be upon him, treated mercy not as softness but as the root of upbringing. A man once saw him kissing his little grandson and said that he himself had ten children and had never kissed one of them. The Prophet, peace be upon him, answered him plainly.

“He who does not show mercy will not be shown mercy.”

The Prophet (peace be upon him), Sahih al-Bukhari (5997); Sahih Muslim (2318) · meaning

He also taught that gentleness takes nothing away from a matter, it adds to it. A home run on fear can look orderly from the outside and still be a cold place to live in.

“Gentleness is not in anything except that it beautifies it, and it is not removed from anything except that it disgraces it.”

The Prophet (peace be upon him), Sahih Muslim (2594a) · meaning

Notice, too, how Allah described His own Messenger, whose whole work was the gathering of hearts. It was mercy that kept people around him, and Allah names what harshness would have cost.

“So by the mercy of Allah you were gentle with them. Had you been harsh and hard-hearted, they would have dispersed from around you.”

Qur’an, Aal-e-Imran 3:159 · meaning

So what needs changing is not your love, which is plainly there, but one habit in the way it comes out, and habits are workable. Keep your expectations clear, because children do better with them. Those who study how children grow find, again and again, that warmth joined to steady and reasonable expectations goes with more confidence and more capability than fear and tight control do. Hold the standard, and let your face be warm while you hold it. Closeness comes back sooner than you would think.

One small step: Today, sit beside one of your children for a few unhurried minutes with no correction and no lesson in it, only to ask how the day went and listen to the answer.