James 1:26 - "Those who consider themselves religious and yet do not keep a tight rein on their tongues deceive themselves and their religion is worthless."
So, if you appear to be religious, you attend church all the time and do religious works, yet you complain, you gossip, your tongue is acidic, your works and religion is worthless. It makes all your spiritual stuff useless in God's eyes. You are active in the work of the Lord but your tongue is completely out of control. James is addressing those who are holy, loving and kind but on their jobs, in their families and whatnot, they are gossiping or listening to gossip with a willing ear. They go on murmuring and complaining. Well...whatever they do, it's worthless.
I won't lie. I have said hurtful things to people in the past. I have said words out of pride, I have lied, I have gossiped, I have said words out of anger. But you cannot take those words back. You can seek to restore these relationships through repentance (saying sorry to them) and they forgive you, but once you say words they cannot be taken back. But even the Lord warns us, "Don't use foul or abusive language. Let everything you say be good and helpful, so that your words will be an encouragement to those who hear them."
A Few Tips
1. Rely on God's Strength
With your own efforts, you will fail - "I can do all things through Christ who strengthens me" - Philippians 4:13
2. Dedicate your heart and tongue to the Lord DAILY
You will face new conversations every single day. Therefore, acknowledging your need for discernment and God's grace will make you more conscious of your words.
3. Read the Word of God Daily
"All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: that the man of God may be perfect, thoroughly furnished unto all good words." - 2 Timothy 3:16-17
4. Seek Counsel - Ask those around you if your words offend them
Ask those around you if your words affect them. Ask them about how you speak and re-evaluate your words. Try to bare in mind that everything you speak will be taken into account. Matthew 12:36
5. Learn to speak words that will encourage, comfort, inspire and edify.
Ask the Lord to guide you to speak words that will honour Him - "Let your speech be always with grace, seasoned with salt, that you may know how to answer every man."
Do you know that your words reflect what is in your heart? What are you putting and feeding into your spirit? In your effects to control your tongue, focus on whatsoever is true, lovely, honest, just, pure and of good report - think on such things. (Philippians 4:8)