قُلْ يَٰعِبَادِىَ ٱلَّذِينَ أَسْرَفُوا۟ عَلَىٰٓ أَنفُسِهِمْ لَا تَقْنَطُوا۟ مِن رَّحْمَةِ ٱللَّهِ إِنَّ ٱللَّهَ يَغْفِرُ ٱلذُّنُوبَ جَمِيعًا
"Say: 'O My servants who have gone to excess against themselves! Do not despair of God's mercy. God forgives all sins.'"
That is, the address begins by calling even those who have gone to excess servants. The description comes after: alladhīna asrafū ʿalā anfusihim.
أَسْرَفُوا۟ عَلَىٰٓ أَنفُسِهِمْ — root س-ر-ف: to overstep the measure, to squander. And the particle ʿalā — "against themselves". That is, the excess is named as something done not to another but to oneself.
لَا تَقْنَطُوا۟ — root ق-ن-ط. This root was analysed in Fuṣṣilat at 41/49: there man, when evil touches him, was described as qanūṭ (one who gives up hope). I rely on that treatment.
The one describes the state, the other forbids that state. I record this as my own reading: the Qurʾān first establishes man's tendency, then issues a command against it.
وَٱتَّبِعُوٓا۟ أَحْسَنَ مَآ أُنزِلَ إِلَيْكُم مِّن رَّبِّكُم — "follow the best of what has been sent down to you from your Lord."
In the eighteenth verse the servants given good news had been described as fa-yattabiʿūna aḥsanahū (those who follow the best of it). Thirty-seven verses later the same expression turns into a command.
أَن تَقُولَ نَفْسٌ يَٰحَسْرَتَىٰ عَلَىٰ مَا فَرَّطتُ فِى جَنۢبِ ٱللَّهِ — "before a soul says, 'Alas for me, for what I failed in on the side of God!'"
حَسْرَة — root ح-س-ر: to strip off what lies over a thing and lay it bare; from this come the senses of weariness and regret. The root was treated in al-Mulk, al-Ḥāqqa and al-Baqara.
فِى جَنۢبِ ٱللَّه — janb: the side. The lexicographers dwell on the meaning of the construction and give renderings such as "in the matter of God's due", "on the side of nearness to Him". I do not impose a firm preference, and I enter the note that the construction carries no bodily sense — the care for tanzīh kept consistently throughout this corpus holds here as well.