Galatians 5:17 is qualified by 1 John 3:9...we cannot do the things that we would...
As has been stated before, 1 John 3:9 speaks of the one whose life is characterized by sin. What that does not say is that we cannot sin. In fact, John makes it absolutely clear that believers sin:
1Jn 3:6 No one who abides in him keeps on sinning; no one who keeps on sinning has either seen him or known him.
1Jn 3:7 Little children, let no one deceive you. Whoever practices righteousness is righteous, as he is righteous.
1Jn 3:8 Whoever makes a practice of sinning is of the devil, for the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the works of the devil.
1Jn 3:9 No one born of God makes a practice of sinning, for God's seed abides in him; and he cannot keep on sinning, because he has been born of God.
1Jn 3:10 By this it is evident who are the children of God, and who are the children of the devil: whoever does not practice righteousness is not of God, nor is the one who does not love his brother. (ESV)
Again, it is about what characterizes one's life, sin or righteousness, and even their feelings towards their sin, not whether or not they sin sometimes or even every day, which every believer does. It's a constant battle, as Paul points out in Romans 7.
We would sin if we could...
We can and do. Note what John says close to the end of his first epistle, which supports and is consistent with everything I have said:
1Jn 5:16 If anyone
sees his brother committing a sin not leading to death, he shall ask, and God will give him life—to those who commit sins that do not lead to death. There is sin that leads to death; I do not say that one should pray for that. (ESV)
That's just speaking of visible sin, not those in the heart or done in private. If that isn't convincing, even though that should put an end to the discussion, there is more:
1Co 3:2 I fed you with milk, not solid food, for you were not ready for it. And even now you are not yet ready,
1Co 3:3 for you are still of the flesh. For while
there is jealousy and strife among you, are you not of the flesh and behaving only in a human way? (ESV)
1Co 5:1 It is actually reported that there is sexual immorality among you, and of a kind that is not tolerated even among pagans, for a man has his father's wife.
1Co 5:2 And
you are arrogant! Ought you not rather to mourn? Let him who has done this be removed from among you.
...
1Co 5:6 Your boasting is not good. Do you not know that a little leaven leavens the whole lump? (ESV)
1Co 6:8 But
you yourselves wrong and defraud—even your own brothers! (ESV)
1Co 8:12 Thus,
sinning against your brothers and wounding their conscience when it is weak,
you sin against Christ. (ESV)
1Co 11:20 When you come together, it is not the Lord's supper that you eat.
1Co 11:21 For in eating,
each one goes ahead with his own meal. One goes hungry,
another gets drunk.
1Co 11:22 What! Do you not have houses to eat and drink in?
Or do you despise the church of God and humiliate those who have nothing? What shall I say to you? Shall I commend you in this?
No, I will not. (ESV)
Gal 2:11 But when Cephas came to Antioch, I opposed him to his face, because
he stood condemned.
Gal 2:12 For before certain men came from James, he was eating with the Gentiles; but when they came he drew back and separated himself, fearing the circumcision party.
Gal 2:13 And the rest of the Jews
acted hypocritically along with him, so that even Barnabas was
led astray by their hypocrisy.
Gal 2:14 But when I saw that
their conduct was not in step with the truth of the gospel, I said to Cephas before them all, “If you, though a Jew, live like a Gentile and not like a Jew, how can you force the Gentiles to live like Jews?” (ESV)
Gal 6:1 Brothers, if anyone is caught in any transgression, you who are spiritual should restore him in a spirit of gentleness.
Keep watch on yourself, lest you too be tempted. (ESV)
Heb 12:1 Therefore, since we are surrounded by so great a cloud of witnesses,
let us also lay aside every weight, and sin which clings so closely, and let us run with endurance the race that is set before us,
...
Heb 12:4
In your struggle against sin you have not yet resisted to the point of shedding your blood.
Heb 12:5 And have you forgotten the exhortation that addresses you as sons? “
My son, do not regard lightly the discipline of the Lord, nor be weary when reproved by him.
Heb 12:6
For the Lord disciplines the one he loves, and chastises every son whom he receives.”
Heb 12:7
It is for discipline that you have to endure. God is treating you as sons. For what son is there whom his father does not discipline?
Heb 12:8
If you are left without discipline, in which all have participated, then you are illegitimate children and not sons. (ESV)
There is much more that could be posted but this is more than sufficient to prove that Christians sin because we still battle the flesh and its desires.
however, the Holy Spirit restrains us.
The Holy Spirit doesn't restrain us but provides a way out of temptation, if we so choose (1 Cor 10:13).