You must withdraw mentally, rather than physically, in your intention, in your devotion, in your spirit. Christ the Lord is a spirit and he demands solitude of the spirit more than of the body–though physical withdrawal can be a benefit when the opportunity offers itself, especially in time of prayer.
To do this, follow the advice of the bridegroom: shut the door and then pray! And what he said, he actually did. He spent nights alone in prayer, not merely hiding from the crowds but even from his disciples and familiar friends. He did indeed take three of his friends with him when the hour of his death was approaching; but the urge to pray drew him apart even from them. You too must act like this when you wish to pray.
Apart from that, the only solitude prescribed for you is that of mind and spirit. You enjoy this solitude if you refuse to share in the common gossip, if you shun involvement in the problems of the hour and set no store by the fancies that attract the masses; if you reject what everyone covets, avoid disputes, make light of losses, and pay no heed to injuries. Otherwise you are not alone even when you are alone. Do you not see that you can be alone in company–and can be in company when you are alone? However great the crowds that surround you, you can enjoy the benefits of solitude if you refrain from curiosity about other peoples’ conduct and shun rash judgment.
Bernard of Clairvaux, On the Song of Songs, Sermon 40, (CF 7, p 202-203)