Hi Roy B,
Some points for reconsideration about the scripture you had referenced.
James 4:17 was in relation to James 4:13-16 about how making plans for tomorrow was akin to evil boasting. The correction was to say," If the Lord will... " to any plans for tomorrow. In the conclusion of verse 17, was to serve as a deterrant for procrastinating from doing anything good today. Keep in mind that just as we are to rely on God to do good tomorrow, so are we to rely on God today to do good since it is obvious we cannot live that christian life without faith in the Son of God as our Enabler.
As for Hebrews 10:26, the sinning willfully when in context is in regards to after having received the knowledge that there is no more sacrifice for sins. That means if a saved believer still believes that there is a necessity for more sacrifice for sins, then he is sinning willfully after that point by denying this truth.
That was why those saved believers that do that are going to get a comeuppance from the Father for treating the blood of the Son as if it was the same as the blood of goats and bulls for them to need a continual sacrifice.
Anyway, I am not sure if smoking fits the bill as a sin. People use that to relax just as people use coffee to wake them up in the morning. Granted, too much smoking and too much caffiene can do the body harm, but you will have to ask Jesus about whethor or not He would want you to stop smoking altogether. It's like alcohol, I would think. A little wine is okay, but in excess, where drunkenness is the result, and that depends on a person's tolerance level as it will be different with every one, then no, it is not okay. If a person cannot drink and remain sober, then they should not drink at all. Some people have trouble staying beneathe the limit in drinking alcohol to stay sober, and so they need His help to not drink at all.
Also, when a believer knows another person is an alcoholic, then they should not drink in front of him or have alcohol breath in being around him afterwards les he cause him to stumble. The same would apply if you are around someone trying to quit smoking; then you should not do it or be around them with smoke on your breath afterwards. Have a breath mint or something. It is the small things that God sees in showing love for one another.
I do not smoke; and the wine I have is like a small glass which is on rare occasion, but generally, I do not like alcohol. FYI
Anyway, in regards to your fear of smoking as a sin, that fear will certainly make it harder to quit smoking since smoking will help you relax that tension in your body. Best bet would be to take it to Jesus Christ in prayer and ask Him for help to cut down on smoking, and if it pleases Him, to lead you to stop smoking altogether. Trust Him to do His work in you.
Philippians 4:5 Let your moderation be known unto all men. The Lord is at hand.
6 Be careful for nothing; but in every thing by prayer and supplication with thanksgiving let your requests be made known unto God. 7 And the peace of God, which passeth all understanding, shall keep your hearts and minds through Christ Jesus.
1 Corinthians 6:8 Nay, ye do wrong, and defraud, and that your brethren. 9 Know ye not that the unrighteous shall not inherit the kingdom of God? Be not deceived: neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor abusers of themselves with mankind, 10 Nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners, shall inherit the kingdom of God. 11 And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God. 12 All things are lawful unto me, but all things are not expedient: all things are lawful for me, but I will not be brought under the power of any.
If you do not like the power that smoking has over you, then trust Jesus to deliver you from its power.
It is a general consensus that smoking is bad for your health, but when in doubt, trust Jesus to lead you in the right way.
Don't live in fear, but live by trusting Jesus Christ to overcome smoking's power for you, and He will.
Psalm 19:12 Who can understand his errors? cleanse thou me from secret faults. 13 Keep back thy servant also from presumptuous sins; let them not have dominion over me: then shall I be upright, and I shall be innocent from the great transgression. 14 Let the words of my mouth, and the meditation of my heart, be acceptable in thy sight, O Lord, my strength, and my redeemer.