God is merciful, he gives food to the sinners and athiests.
So no matter what crime u do, God will not send a lightening bolt to kill you. He has given us free will as long as we r alive. We can do whatever, once we do tho. Hell and heaven await.
You think smoking etc lets u have youth? Gives u diseases. And abt low confidence, that is just ur thinking. Even if u get a nice french haircut, u will feel low self confidence and will go out to buy expensive clothes, then jewlery, then cars. Even then if someone ignores u or someone cooler or richer comes, u will feel low confidence.
Before giving up on ur hair. Try to follow sikhi to the fullest. Do ardaas, waheguru ji if there is any faida/benefit for keeping hair or following sikhi, please let me know. Then for a month follow sikhi to the fullest. Wake up at amritwela. Wear a beautiful dastaar/dumaala.Do paath and simran. Go train at the gym. Speak only truth. Visit gurudwara. Do seva. Do honest work. Listen to kirtan. Learn an instrument. Read sikh books by Raghbir singh bir or bhai vir singh.
Do u even know why we keep hair? What is the purpose of religion of sikhi? Find out. Do this for a month, if u get no benefit, then do whatever u want.