Read carefully

The verses:

“Blessed is the man that walketh not in the counsel of the ungodly, nor standeth in the way of sinners, nor sitteth in the seat of the scornful. But his delight is in the law of the LORD; and in His law doth he meditate day and night. And he shall be like a tree planted by the rivers of water, that bringeth forth his fruit in his season; his leaf also shall not wither; and whatsoever he doeth shall prosper.” (Psalms 1:1-3)

This is what I understand*

The Conditions to be ‘fulfilled’ first:

(i)Blessed is the man that walketh not in the counsel of the ungodly, (e.g. don’t follow the wrong worldly advice of those who are against God)
(ii) nor standeth in the way of sinners (e.g. doesn’t hang out in sinful places such as a prostitution den nor does the sinful habits such as getting drunk, committing adultery, joining gangsters, etc.)
(iii) nor sitteth in the seat of the scornful (e.g. doesn’t take part in ‘mocking others’ especially those placed in authority, making obscene remarks against rulers of the country instead of a decent remark should it be wrong etc.)
(iv) But his delight is in the law of the LORD (e.g. one who is happy to read His Words and keep His commandments);
(v) and in His law doth he meditate day and night (e.g. one who constantly considers God’s Word seriously and often thinks about how to practice it in his daily life).

Then, the promise ‘follows’:

(i) And he shall be like a tree planted by the rivers of water (e.g. such a one will be good and clean as the waters cleanse both within and without of a tree),
(ii) that bringeth forth his fruit in his season (e.g. his acts will benefit others around him just like the fruit of a tree nourishes others including his enemies);
(iii) his leaf also shall not wither (e.g. his abilities to absorb God’s Light of knowledge & wisdom remains just like a leaf’s ability to absorb sunlight is retained as long as it’s alive);
(iv) and whatsoever he doeth shall prosper (e.g. prosper according to God’s Will which does ‘not’ necessarily include ‘money’ but may necessarily include persecution of some sort as 2 Timothy 3:12 states –> to the extent —> some may face even ‘martyrdom’ which is a ‘good thing**’, a privilege given only to a few):

“In fact, everyone who wants to live a godly life in Christ Jesus will be persecuted,” (2 Timothy 3:12)

*My interpretation of this passage is opinion based based on what I understand of it (the e.g.’s).

**A ‘suffering’ or a ‘painful’ thing may be a ‘good thing’ according to His Will but only if it’s done in Him as these set of verses reveal (note the words “suffer” and “God’s Will” below):

“For it is better, if it is God’s will, to suffer for doing good than for doing evil.” (1 Peter 3:17)

“But how is it to your credit if you receive a beating for doing wrong and endure it? But if you suffer for doing good and you endure it, this is commendable before God.” (1 Peter 2:20)

“So then, those who suffer according to God’s will should commit themselves to their faithful Creator and continue to do good.” (1 Peter 4:19)

During the ‘tribulation’:

“Then they will deliver you to tribulation, and will kill you, and you will be hated by all nations because of My name.” – Lord Jesus Christ (Matthew 24:9)

“Then one of the elders answered, saying to me, “These who are clothed in the white robes, who are they, and where have they come from?” I said to him, “My lord, you know.” And he said to me, “These are the ones who come out of the great tribulation, and they have washed their robes and made them white in the blood of the Lamb.” (Revelation 7:13 – 14)

“and they did overcome him (Satan) because of the blood of the Lamb, and because of the word of their testimony, and they did not love their life — unto death;” (Revelation 12:11)

An example:

‘I know where you dwell, where Satan’s throne is; and you hold fast My name, and did not deny My faith even in the days of Antipas, My witness, My faithful one, who was killed among you, where Satan dwells.‘ – Lord Jesus Christ (Revelation 2:13)

P/S:

You cannot quote just the ‘promise’ without “fulfilling” the ‘conditions’ first.