Boldness in saying what God would say comes from boldness in doing what God would do

Many shy away from boldly speaking as the Lord would in their place because they feel guilty for not living up to what they believe is best. The guilt you feel when speaking hypocritically is not an excuse not to speak up. Rather, it is a call to become capable of speaking as the Lord would in your place without hypocrisy through first doing what he would in your place, or in other words, repenting of your sins.¹

If you want more courage in saying what the Lord would say, change your life to do more of what the Lord would do:

Let thy bowels also be full of charity towards all men, and to the household of faith, and let virtue garnish thy thoughts unceasingly; then shall thy confidence wax strong in the presence of God; and the doctrine of the priesthood shall distil upon thy soul as the dews from heaven. (D&C 121:45)

18 My little children, let us not love in word, neither in tongue; but in deed and in truth.
19 And hereby we know that we are of the truth, and shall assure our hearts before him.
20 For if our heart condemn us, God is greater than our heart, and knoweth all things.
21 Beloved, if our heart condemn us not, then have we confidence toward God.
22 And whatsoever we ask, we receive of him, because we keep his commandments, and do those things that are pleasing in his sight. (1 John 3)

Those who act as they believe the Lord would at all times have no lack of courage in saying what he would.² As you increase your heed and obedience to what you believe is best, you will find that your confidence in inviting others to be better will increase.