Should I Use Positive Thinking to Be Healthier?

Yes.

Mere positive thinking will not let you escape hell; only Jesus can save you from eternal torment for your sins.  However, the Bible actually commands positive thinking!

Philippians 4:8:
Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things.

God does not choose what you set your mind on.  If He did, He would be unjust to command you to think on positive things.  The fact that you are commanded to think on good things proves that you can do it no matter how horrible the circumstances appear.  There is no such thing as being so overwhelmed that it is literally impossible for you to focus on good things if you choose to do so.

If you choose to set your mind on God, you set yourself up for peace.

Isaiah 26:3:
Thou wilt keep him in perfect peace, whose mind is stayed on thee: because he trusteth in thee.

Paul said that renewing your mind with the Word will transform you:

Romans 12:2:
And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God.

However, thinking God’s positive thoughts is more powerful than conjuring up your own positive thoughts.  For example, it’s been suggested for a long time in the world that you focus on the phrase, “Every day in every way I’m getting better and better.”  While that is a vast improvement on “Every day in every way I’m getting worse and worse and I’m really afraid,” it lacks God’s power because it contains no basis for why you should expect to be better.  When you think God’s thought, “By Jesus’ stripes I was healed,” you are thinking about Scripture that is backed by a faithful God who cannot lie.  There is power in that.

In fact, I definitely discourage you from using non-Christian but nice-sounding “affirmations” rather than confessing who you are and what you have in Christ.  The kind of positive thinking you need to do is Word-based, not just willpower-based or mind-based.

Thanksgiving is a related concept.  There are so many verses telling us to give thanks to the Lord that I won’t list them all here.  Here are some key New Testament ones:

Ephesians 5:20:
Giving thanks always for all things unto God and the Father in the name of our Lord Jesus Christ;

1 Thessalonians 5:18:
In every thing give thanks, for this is the will of God in Christ Jesus concerning you.

Hebrews 13:15:
By him therefore let us offer the sacrifice of praise to God continually, that is, the fruit of our lips giving thanks to his name.

Colossians 3:17:
And whatsoever ye do in word or deed, do all in the name of the Lord Jesus, giving thanks to God and the Father by him.

It’s impossible to complain and give thanks at the same time.  These commands are another way to ensure positive thinking (thanksgiving) rather than negative thinking (complaining).  You won’t like the fruit of complaining.

Psalm 77:3:
I remembered God, and was troubled: I complained, and my spirit was overwhelmed.  Selah.

Psalm 142:2-3:

I poured out my complaint before him; I shewed before him my trouble.

When my spirit was overwhelmed within me, then thou knewest my path. In the way wherein I walked have they privily laid a snare for me.