If we are infused and made holy in justification --how does this tie in with sin in the believer?

For though during this mortal life, men, however holy and just, fall at times into at least light and daily sins, which are also called venial, they do not on that account cease to be just, for that petition of the just, forgive us our trespasses,[62] is both humble and true; for which reason the just ought to feel themselves the more obliged to walk in the way of justice, for being now freed from sin and made servants of God,[63] they are able, living soberly, justly and godly,[64] to proceed onward through Jesus Christ, by whom they have access unto this grace.[65]

For God does not forsake those who have been once justified by His grace, unless He be first forsaken by them.  (from Council of Trent--on justification here: http://www.ewtn.com/library/councils/trent6.htm)

One still has to learn obedience by trials etc, because even Christ who was sinless had to do this.  More on this from Trent: 

 For even Christ Himself, as the Apostle says, whereas he was the Son of God, he learned obedience by the things which he suffered, and being consummated, he became to all who obey him the cause of eternal salvation.[67] 

For which reason the same Apostle admonishes those justified, saying:  

Know you not that they who run in the race, all run indeed, but one receiveth the prize? 

So run that you may obtain.
I therefore so run, not as at an uncertainty; I so fight, not as one beating the air, but I chastise my body and bring it into subjection; lest perhaps when I have preached to others, I myself should become a castaway.[68] 

So also the prince of the Apostles, Peter:
Labor the more, that by good works you may make sure your calling and election.
For doing these things, you shall not sin at any time.[69]

One person put it this way:

Does our salvation depend on keeping every single command all the time? No. For by grace you have been saved through faith.

In the Protestant framework, this verse means that faith alone is sufficient to save us, because through this belief God irrevocably imputes extra nos the obedience of Christ to us, so that it doesn’t ultimately matter how much or how little we keep His commands the rest of our earthly lives. Our degree of sanctification at death does not determine whether we are saved or not. We’re covered.

But in the Catholic understanding, this verse means that God gives us the grace through the gift of living faith (faith informed by agape) which is the righteousness to which the law points, and by which we keep His commands. Grace is not an alternative to keeping the law; it is precisely that divine gift through which the law is truly fulfilled in us.

from comment 130 http://www.calledtocommunion.com/2012/08/imputation-and-paradigms-a-reply-to-nicholas-batzig/

But the problem I see with this is it doesn't say For by faith you have been saved through grace.   But I suppose he means that when grace comes--in Baptism then sanctifying grace gives faith, hope , love. This living faith has this love  in which  righteousness is based since love is the fulfilling of the law--the law basically shows how love is to be practiced. Love is the fulfilling of the law. Venial sins do not take away our love , but mortal ones do.

Trent says more on this:

CHAPTER XIVTHE FALLEN AND THEIR RESTORATIONThose who through sin have forfeited and received grace of justification, can again be justified when, moved by God, they exert themselves to obtain through the sacrament of penance the recovery, by the merits of Christ, of the grace lost.[82]

For this manner of justification is restoration for those fallen, which the holy Fathers have aptly called a second plank after the shipwreck of grace lost.[83]

For on behalf of those who fall into sins after baptism, Christ Jesus instituted the sacrament of penance when He said:

Receive ye the Holy Ghost, whose sins you shall forgive, they are forgiven them, and whose sins you shall retain, they are retained.[84]

Hence, it must be taught that the repentance of a Christian after his fall is very different from that at his baptism, and that it includes not only a determination to avoid sins and a hatred of them, or a contrite and humble heart,[85] 

but also the sacramental confession of those sins, at least in desire, to be made in its season, and sacerdotal absolution, as well as satisfaction by fasts, alms, prayers and other devout exercises of the spiritual life, not indeed for the eternal punishment, which is, together with the guilt, remitted either by the sacrament or by the desire of the sacrament, but for the temporal punishment which, as the sacred writings teach, is not always wholly remitted, as is done in baptism, to those who, ungrateful to the grace of God which they have received, have grieved the Holy Ghost[86] 

and have not feared to violate the temple of God.[87]
Of which repentance it is written: 

Be mindful whence thou art fallen; do penance, and do the first works;[88] and again, The sorrow that is according to God worketh penance, steadfast unto salvation;[89] and again, Do penance, and bring forth fruits worthy of penance.[90]

also  from comment 242 here: http://www.calledtocommunion.com/2012/09/i-fought-the-church-and-the-church-won/comment-page-5/#comment-38522

The baptized person has infused agape, and is therefore righteous, even though he or she simultaneously retains dispositions toward particular sins. Infused agape does not mean that the person is perfect in all his habits. (See “Imputation and Paradigms“.) It does mean that he loves God as Father, from the heart, by the Holy Spirit’s pouring out of this gift into his heart (Rom 5:5). This is how the believer can be truly righteous now, and yet be perfected still at death before entering heaven. Among other things, at death all remaining vices are removed.