When one is afraid, the fear itself makes it more likely for that which is feared to occur, r'l.
Think of a person walking through a very dangerous part of the city. There are bandits and murderers on every corner. If the person walks erect, confidently, and without fear, the bandits will leave him alone. They figure that if he is so confident, perhaps he has a weapon on him. Maybe he really has no reason to fear. In contrast, think of a person who is afraid and timid by nature. When he hears someone walking behind him in this dangerous neighborhood, his pace quickens and he nervously looks behind him to see if that person wants to harm him. He is calling attention to himself, declaring that he is afraid, and that encourages the bandits to steal from him and harm him.
The Maharal says that when you are afraid of something, it means that you consider the matter greater than you and are subjugated to it. This gives the matter strength to rule over you.
The Maharal (Chidushei Agodos Bava Metzia 33) writes, “The Rishonim say: When one is afraid that he might fall, that fear itself can cause him to fall.
"Place a wooden plank across a river and walk across it. You will likely fall into the river. Now place the plank on the ground and walk over it. You won't fall off. Because when you are above a river, you are afraid of falling, which causes it to happen. This is also true when one fears becoming poor. Thinking about this causes it to happen... As it states in Iyov (3:25), 'That which I was afraid of, came to me.'”
The Gemara (Bava Metzia 33.) says, המקיים כל כך לידי בא כך בעצמו, "Whoever is always afraid that he might become poor, he will become poor." The Maharal explains, "When one is afraid of poverty, he is giving poverty the ability to affect him and to reach him... When one is afraid of something, he is making himself small before that matter that he is afraid of, which gives the matter strength to affect him. This is especially true regarding poverty. When one is afraid of poverty, the thought causes an effect on the person, and he will receive the poverty."
The Gemara (Brachos 60.) also tells us that fear can cause troubles to occur. The Gemara tells that Rav Hamnuna once heard his student Rav Yehudah bar Nasan moan. Rav Hamnuna told him that the moans are bringing about difficulties unto himself, as it states (Iyov 3:25), יבא יגרתי ואשר ויאתני פחדתי פחד כי לי, ‘I was afraid, and it happened. The matter I was worried about came to me.”
The Riv’a (from the Baalei HaTosfos) teaches that Yaakov lost his protection because of his fear, which is why the malach was able to smite Yaakov.
2.
Seforno explains that as long as Yaakov Avinu thought about Hashem, he was protected. The moment he lost focus, the malach could smite him on the thigh. "Because of Yaakov's constant deveikus in Hashem through his thoughts and speech, the malach couldn’t harm him. But when the malach told him about the nation's future sins (עמו במדריכי העתיד החטא הודיעו), this upset Yaakov and his deveikus ceased, and [that's when the angel hit him and] his thigh got dislodged."
The Zutchke Rebbe zt'l offered another explanation of how Yaakov was wounded, even after Hashem promised to protect him. He explains that the Sar of Esav didn’t harm Yaakov. He couldn’t because Hashem was protecting Yaakov. Instead, Yaakov hurt himself. Wrestling necessitates strenuous, unusual body movements. As Yaakov battled with the angel, his thigh dislodged because of those maneuvers.
The Zutchka Rebbe explains the pasuk as follows: לו יכול לא כי וירא, "The malach saw that he couldn't harm Yaakov" because Hashem was protecting him. ירכו בכף ויגע, "So he touched Yaakov's thigh." This means that he held onto Yaakov. That he could do. He didn’t have permission to harm Yaakov but could grab Yaakov. If Yaakov had realized that angel couldn’t do more than that, he wouldn't have been afraid and wouldn’t have reacted. However, Yaakov was afraid and struggled and wrestled with the angel, and יעקב ירך כף ותקע עמו בהאבקו, "Yaakov's thigh dislodged, due to his own actions and movements in the struggle."
3.
It states (Mishlei 28:26) ימלט הוא בחכמה והולך כסיל הוא בלבו בוטח, "One who trusts his heart is a fool, and one who walks with wisdom will escape." The Ralbag explains, "The fool doesn't make plans and strategies before he sets out to attain his needs. He trusts his heart and instincts that he will take the correct actions. Such a person fails. However, when one goes with wisdom, he will be saved from all bad and obstacles placed in his way because he planned how to deal with these situations and obstacles when confronted with them."
The Metzudos Dovid says בחכמה והולך means that if he is wise and knows that he should trust in Hashem, he will succeed.
