A little honesty in life may help remold you into a better version of yourself. It goes a long way in making life easier for you more than you could ever imagine.
There's honor in honesty. Sometimes you need to let him know you really won't be able to come to his birthday party than call him when the party is over with tons of excuses. Sometimes I think it's better to let her know you don't love her and shake her heart with that fierce sounding but honest words for that moment than deceive in her into believing you're in love with her and break her heart in the middle of nowhere when she's love-drunk. Sometimes It's just but reasonable to let him know you're mad at him than secretly hold grudge against him faking a smile every now and then.
Apart from being honest with people, be honest with yourself too. Don't fake enjoying that stuff that you're bored of. Stop hailing and praising yourself for what you're not which you've made them believe you are. She doesn't love you anymore and you know it, then stop the emotional violence against yourself. A personal punishment that's coming from you to you through her.
Be honest, not just with everyone, but with yourself too.
Honesty is a rare habit that's not common in peoples lives. It can rarely be found in this current world of ours