If the Sacrament of Confirmation gives the gift of fortitude, why are we still afraid to do the right thing? If the Sacrament of Confirmation gives all the Gifts of the Holy Spirit, including the gift of fortitude, why are we still afraid to do the right thing?
If the Sacrament of Confirmation gives the gift of fortitude, why are we still afraid to do the right thing?

The short answer is that our human nature is still damaged because of sin, both original sin and actual sins.

Before getting into more details of this question, let us see what the Catechism of the Catholic Church has to say on the effects of confirmation.


  The Effects of Confirmation
  
  1302 It is evident from its celebration that the effect of the sacrament of Confirmation is the full outpouring of the Holy Spirit as once granted to the apostles on the day of Pentecost.
  
  1303 From this fact, Confirmation brings an increase and deepening of baptismal grace: 
  
  
  it roots us more deeply in the divine filiation which makes us cry, "Abba! Father!";115 
  it unites us more firmly to Christ; 
  it increases the gifts of the Holy Spirit in us; 
  it renders our bond with the Church more perfect;116 
  it gives us a special strength of the Holy Spirit to spread and defend the faith by word and action as true witnesses of Christ, to confess the name of Christ boldly, and never to be ashamed of the Cross:117
  
  
  Recall then that you have received the spiritual seal, the spirit of wisdom and understanding, the spirit of right judgment and courage, the spirit of knowledge and reverence, the spirit of holy fear in God's presence. Guard what you have received. God the Father has marked you with his sign; Christ the Lord has confirmed you and has placed his pledge, the Spirit, in your hearts.118
  
  1304 Like Baptism which it completes, Confirmation is given only once, for it too imprints on the soul an indelible spiritual mark, the "character," which is the sign that Jesus Christ has marked a Christian with the seal of his Spirit by clothing him with power from on high so that he may be his witness.119
  
  1305 This "character" perfects the common priesthood of the faithful, received in Baptism, and "the confirmed person receives the power to profess faith in Christ publicly and as it were officially (quasi ex officio)."120


This sacrament completes that of baptism and confirms us in grace.

Original sin and actual mortal sin destroys sanctifying grace in our souls. Baptism restores sanctifying grace to the souls. However our human nature remains damaged due to original sin and actual sin. 


  Concupiscence.- This rebellion of the lower appetite transmitted to us by Adam is an occasion of sin and in that sense comes nearer to moral evil. However, the occasion of a fault is not necessarily a fault, and whilst original sin is effaced by baptism concupiscence still remains in the person baptized; therefore original sin and concupiscence cannot be one and the same thing, as was held by the early Protestants (see Council of Trent, Sess. V, can. v).


Thus actual sins weaken our abilities to resist future evils and temptations and induces us to commit greater sins; thus making it harder to stay in the state of sanctifying grace.

Although sanctifying grace has been restored to our souls through the sacrament of baptism and we are strengthened through the graces received in the sacrament of confirmation, we must still work out our individual salvation. The sacrament aid us in becoming saints, but we must still do our part in fortifying our souls through all means the Church places at our disposal, such as the sacraments of the Eucharist and Confession. 

Sin is terrible and continuously weekends our human nature while the sacraments do the opposite. The Catechism of the Catholic Church can shed some light on this:


  1714 Man, having been wounded in his nature by original sin, is subject to error and inclined to evil in exercising his freedom.
  
  1854 Sins are rightly evaluated according to their gravity. The distinction between mortal and venial sin, already evident in Scripture,129 became part of the tradition of the Church. It is corroborated by human experience.
  
  1855 Mortal sin destroys charity in the heart of man by a grave violation of God's law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him.
  
  Venial sin allows charity to subsist, even though it offends and wounds it.
  
  1856 Mortal sin, by attacking the vital principle within us - that is, charity - necessitates a new initiative of God's mercy and a conversion of heart which is normally accomplished within the setting of the sacrament of reconciliation:
  
  When the will sets itself upon something that is of its nature incompatible with the charity that orients man toward his ultimate end, then the sin is mortal by its very object . . . whether it contradicts the love of God, such as blasphemy or perjury, or the love of neighbor, such as homicide or adultery. . . . But when the sinner's will is set upon something that of its nature involves a disorder, but is not opposed to the love of God and neighbor, such as thoughtless chatter or immoderate laughter and the like, such sins are venial.
  
  1872 Sin is an act contrary to reason. It wounds man's nature and injures human solidarity.


The sacrament of confirmation helps us to live our lives as saints, but we must at the same time cooperate with those very same graces received on the day we were confirmed by our local ordinaries (bishops). 


  Actual sin primarily consists in a voluntary act repugnant to the order of right reason. The act passes, but the soul of the sinner remains stained, deprived of grace, in a state of sin, until the disturbance of order has been restored by penance. This state is called habitual sin, macula peccati. reatus culpæ (I-II:87:6).


Since our human nature has been damaged by original sin and actual sin; the sacraments of baptism and confirmation are there to restore us to God’s sanctifying grace and to give us the graces necessary to remain holy. We must strive to do our part and be docile to the Spirit of Holiness and cooperate with the graces infused into our soul by the Holy Spirit in the sacrament of confirmation.

In closing, we must also keep in mind that the Devil is always seeking way to make us fall, whether that be or through fear of some other motivation.