Jeremiah 29 is a promise, written for the Israel who are of God, those who are being obedient to God's command for them to go to Babylon for a period of time being 70 years, those who are God's people still, whom He will bless wherever they go.
There were some jews who fought against God's decree of destruction on Israel, refusing to submit to the plan of God, they were rebels against the Lord. And they were destroyed. Some went to Egypt and God said those going to Egypt would die there.
Jeremiah 29, is clearly addressed to those who are in Babylon as the Lord ordained for them to go. but some were to remain in the land of Israel.
10 For thus says the Lord: After seventy years are completed at Babylon, I will visit you and perform My good word toward you, and cause you to return to this place. 11 For I know the thoughts that I think toward you, says the Lord, thoughts of peace and not of evil, to give you a future and a hope. 12 Then you will call upon Me and go and pray to Me, and I will listen to you. 13 And you will seek Me and find Me, when you search for Me with all your heart. 14 I will be found by you, says the Lord, and I will bring you back from your captivity; I will gather you from all the nations and from all the places where I have driven you, says the Lord, and I will bring you to the place from which I cause you to be carried away captive.
Jeremiah 42 is for the rebels who refuse to be submitted to the judgement of God.
18 “For thus says the Lord of hosts, the God of Israel: ‘As My anger and My fury have been poured out on the inhabitants of Jerusalem, so will My fury be poured out on you when you enter Egypt. And you shall be an oath, an astonishment, a curse, and a reproach; and you shall see this place no more.’
19 “The Lord has said concerning you, O remnant of Judah, ‘Do not go to Egypt!’ Know certainly that I have admonished you this day. 20 For you were hypocrites in your hearts when you sent me to the Lord your God, saying, ‘Pray for us to the Lord our God, and according to all that the Lord your God says, so declare to us and we will do it.’
21 And I have this day declared it to you, but you have not obeyed the voice of the Lord your God, or anything which He has sent you by me. 22 Now therefore, know certainly that you shall die by the sword, by famine, and by pestilence in the place where you desire to go to dwell.”
If you can claim out of the context a promise for yourself in Jeremiah 29, why not many more Jeremiah scriptures such as not going to Egypt?
For today, you in the church, believe the words of Christ and the apostles , of which are also many comforting words of promise and consolations. The OT words here in Jeremiah 29 were comforting and consoling words of promise to set into Israel's heart that God had planned a good future and a hope for them. God had proven His word true to Israel in bringing about the captivity in Babylon, so they knew God was the LORD God , all powerful and true to His word spoken by His prophet Jeremiah, so they could take comforting assurance that what Jeremiah had spoken was the word of the Lord for them and God would bring it to pass.
The REBELS though, will never be obedient to God or believe what God has said, and God will destroy them everyone. Rebels believe the only fate is what they make and they do not love the Lord's working, they dont love God and don't love His judgements, His word, they dont love what He says.