I'm not Arab. But I'm learning it. And as I read Quran, in a lot of places it says "Yahdi mayyasha" meaning that God will guide one who wants. The question is does "yasha" refer to God? or to the person who chooses and wants to be guided? Because if it refers to God, the meaning becomes sooooo unjust, and if it's about the guided's will, then things are up to us. As much as I see translations and interpretations, they say "yasha" refers to God. Is there an Arabic grammatical rule that I'm not aware of that forces this interpretation?
As you pointed out, the verb "yasha" could refer back to God (i.e yahdi, He guides). But it is also logically valid that it refers to the one "who wants" (to be guided).
The most famous interpretation is the first, i.e that God guides whom He (God) wills.
سَيَقُولُ السُّفَهَاء مِنَ النَّاسِ مَا وَلاَّهُمْ عَن قِبْلَتِهِمُ الَّتِي كَانُواْ عَلَيْهَا قُل لِّلّهِ الْمَشْرِقُ وَالْمَغْرِبُ يَهْدِي مَن يَشَاء إِلَى صِرَاطٍ مُّسْتَقِيمٍ
THE WEAK-MINDED among people will say, "What has turned them away from the direction of prayer which they have hitherto observed?" Say: "God's is the east and the west; He guides whom He wills onto a straight way
2:142 (Translation Muhammad Asad)
Muhammad Asad also adds in his commentary to this verse that it could refer to the one who wants to be guided:
Or: "He guides onto a straight way him that wills [to be guided]" Muhammad Asads comment on verse 2:142
He continues explaining his reasons for this interpretation in verse 14:4:
وَمَا أَرْسَلْنَا مِن رَّسُولٍ إِلاَّ بِلِسَانِ قَوْمِهِ لِيُبَيِّنَ لَهُمْ فَيُضِلُّ اللّهُ مَن يَشَاء وَيَهْدِي مَن يَشَاء وَهُوَ الْعَزِيزُ الْحَكِيمُ
AND NEVER have We sent forth any apostle otherwise than [with a message] in his own people's tongue, so that he might make [the truth] clear unto them;  but God lets go astray him that wills [to go astray], and guides him that wills [to be guided] -for He alone is almighty, truly wise.
In his note to that verse he explains:
Or: "God lets go astray whomever He wills, and guides whomever He wills". All Qur'anic references to God's "letting man go astray" must be understood against the background of 2:26-27 - "none does He cause to go astray save the iniquitous, who break their bond with God" (regarding which latter expression, see surah 2, note 19): that is to say, man's "going astray" is a consequence of his own attitudes and inclinations and not a result of an arbitrary "predestination" in the popular sense of this word (cf. surah 2, note 7). In his commentary on the above verse, Zamakhshari stresses this aspect of free choice on the part of man and points out that "God does not cause anyone to go astray except one who, as He knows, will never attain to faith; and He does not guide anyone aright except one who, as He knows, will attain to faith. Hence, the [expression] 'causing to go astray' denotes [God's] leaving [one] alone (takhliyah) and depriving [him] of all favour, whereas [the expression] 'guidance' denotes [His] grant of fulfilment (tawfiq) and favour .... Thus, He does not forsake anyone except those who deserve to be forsaken, and does not bestow His favour upon anyone except those who deserve to be favoured." Commenting on the identical phrase occurring in 16: 93, Zamakhshari states: "[God] forsakes him who, as He knows, will [consciously] choose to deny the truth and will persevere in this [denial]; and ... He bestows His favour upon him who, as He knows, will choose faith: which means that He makes the issue dependent on [man's] free choice (al-ikhtiyar), and thus on his deserving either [God's] favour or the withdrawal of [His] aid ... and does not make it dependent on compulsion [i.e., predestination], which would rule out [man's] deserving anything of the above."(Quran Ref: 14:4 )