To understand your state of mind, you should introspect & see what are the reasons that your mind becomes stressful, tense & disturbed. If you throw a small stone in the calm, peaceful pond of water then ripples are generated in the pond. If you try to stop those ripples with your hand or any other means, more & more ripples are generated. Same is the case with our mind. When our peaceful mind is disturbed due to any reason in our life then trying to undo that situation is very difficult. So we should try to analyze in our life, what was the reason that caused those ripples in the first place in your peaceful mind. Also, in future you should try to stop such situations in your life.
The main reason why our mind remains disturbed is that we are not satisfied with our present state. If you have something, you are not happy with it. In your relations you may not have harmonious relations with your near & dear ones. If you have a particular amount of money with you, it seems less to you & you want more of it. Same is that, you have Greed for other things & don’t know how to enjoy the one’s you are already having.
To be peaceful at mind, the first & foremost thing is to be satisfied with what you have at present & try to make harmonious relations with the people all around you. Once you are able to do that then you shall find that you are better off & much peaceful. Just try to appreciate the positive aspects in your life & avoid pondering over negatives in life & that shall make you more peaceful. Also try to control your desires as unfulfilled desires are the ones which keep our mind disturbed.
Below are few verses of Bhagwat Gita which teaches, how a person of peaceful, unwavering and undisturbed mind behaves in life:
samudram apah pravishanti yadvat
tadvat-kama yam pravishanti sarve
sa shantim-apnoti na kama-kaami” (Bhagwat Gita: Chapter II verse 70)
"As the waters (of different rivers) enters the Great Ocean, which though full on all sides remains undisturbed, like wise a person who is not disturbed by the incessant flow of desires - can alone achieve peace, and not the man who runs after these desires & strives to satisfy such desires."
tulya-nindatma-samstutih" (Bhagwat Gita: Chapter Fourteen verse 24)
Sri Krishna said to Arjuna: One who is situated in the Self (soul) and behaves same during happiness and distress; one who looks upon a lump of earth, a stone and a piece of gold with an equal eye; one who is equal toward the desirable and the undesirable; one who is situated equally well in praise and blame is the one who is of steady mind.
gunatitah sa ucyate" (Bhagwat Gita: Chapter Fourteen verse 25)
Sri Krishna said to Arjuna: One who is steady in honor and dishonor; one who treats alike both friend and enemy; and who has renounced all material activities—such a person is said to have transcended the three modes of nature & lives in a peaceful state of mind.
So if you want to have peace of mind in life, you need to be satisfied with what you have at present in life & should not hanker after those desirous objects which are beyond your reach. Just try to make harmonious relations with the people all around you & you shall feel peaceful & happy in life. Other than that turning your mind's attention towards God or Spiritual practices like listening to devotional songs (Bhazans), reading spiritual literature, chanting religious mantras, doing Yoga etc. shall also make your mind peaceful & the "Key to Happiness lies in a Peaceful Mind only".
Jai Sri Krishna