Have you ever realized too late that you missed an opportunity to witness? Most of us have done that. What do you do about it? I suggest that you pray.
Of course, the enemy of our souls is doing everything he can to debilitate you with guilt feelings. You need to confess that as sin. Agree with God and thank Him for His forgiveness. Then ask Him to help you be alert for the next opportunity. The enemy may be telling you that God will never trust you again. But Satan is a liar. In fact, I have noticed that God uses missed chances to bear witness, to alert me to another opportunity coming up.
Another important factor in being alert for opportunities to share a witness for Christ is regularly praying for lost people. Have you made a prayer list of lost people God has brought into your life. This will help you be ready the next time God opens a door for you to tell someone about Jesus.
But the most important thing you can do to be ready to give a witness is to pray without ceasing. There are a number of applications to this command given us in 1 Thessalonians 5:17. But the most pertinent to this point is praying for people all around you. Develop the habit of praying for everyone you see when you walk down the street, into a store, into your office or the bank, when you are filling your car with gas or returning books to the library. As you pray you will begin to see people as God sees them. You will be more alert to their longings and hurts and needs. And you will be more in tune with the Holy Spirit as He prompts you to speak and gives you the words to say.