In The School of Prayer (2)

"Be unceasing in prayer [praying perseveringly]; (1 Thessalonians 5:17 AMP)
"...do not forbid or hinder speaking in [unknown] tongues." ( 1 Corinthians 14:39 AMP)
For a man who will not pray in tongues ceaseless prayer as commanded by scriptures is difficult if not impossible; the man heeding that command by praying only in his understanding has signed up for vain repetition in prayer.
At the highest peak of your spirituality, your mind is still limited in Knowledge and understanding; no matter how informed you are about the things of God, the summary of your grasp where the vast knowledge of God is concerned is summed up by ICor. 13:12 - "you Know in part". Hence, if all you are "drawing" from is your mind (which has limitation), you will likely get stuck at a point; and that might just be the reason you could not persevere in prayer having been frustrated by your limitation and perceived boredom.
Please pray in your understanding (ICor. 14:14) for it is profitable, but pray more in tongues. You cannot pray amiss praying in tongues, neither can you enter vain repetition. When you pray in tongues, you are drawing from the limitless knowledge of God directly from the mind of the Spirit; you are praying beyond what you know; you are partnering with God to pray out His will.
Praying in tongues is the"lifeline"of ceaseless prayer. I believe the reason they were not commanded to pray without ceasing under the old covenant is because, then they were not born of the spirit and thus lack the capacity to pray in the spirit. Speaking in tongues is an exclusive characteristic of the New Testament.
Ever wondered how possible it is to truly pray without ceasing, then do it praying in tongues; you will discover that none of God's command to the believer is grievous (burdensome). Do I meet you praying?