References – AN 11.12: With Mahānāma (2nd) 
“I find it very difficult to stay with the breath. After 15-20 minutes, the mind will be very bored with the breath and cannot stay with it. Could you advise what is a good meditation for an easily restless mind?” Answered by Ajahn Pasanno. [Mindfulness of breathing] [Restlessness and worry] // [Attitude] [Desire] [Meditation/Techniques] [Investigation of states] [Gladdening the mind] [Happiness]
Sutta: MN 118: Ānāpānasati Sutta
Suttas: AN 11.1, AN 11.12, and SN 55.40 illustrate the causal chain between happiness and samādhi.