My problem is lack of concentration during prayers. I just want to finish quickly and think about what to do next. What is the remedy, if there is any?
St. Paul said in the Holy Book of 1 Corinthians 14:19 "Yet in the church I had rather speak five words with my understanding, that by my voice I might teach others also, than ten thousand words in an unknown tongue." This verse implies that we should care about the quality of prayers more than the quantity of prayers. I recommend discussing with your spiritual father how many hours from the Agpyea you need to pray and how many Holy Psalms per each hour. I am sure this will regulate your prayer time better.
Prayer is a way of life and is a continuous warfare against the devil. This struggle you are facing is nothing but a temptation from the devil to stop you from doing the thing he hates most, which is praying and having this personal relationship with God.
If you dedicate a specific length of time daily for your prayers during which you put everything else in your life on hold and concentrate on prayers, you will not feel that prayers are time consuming. You said you enjoy praying the Agpeya so try again to concentrate on the words you are praying and if any distracting thought comes to mind,, instead of entertaining these thoughts hand them to God in your prayers and ask Him to take charge of your day and bless your time so it would be more fruitful and you will be able to accomplish more in your day without feeling that praying is time consuming.
It is very good to have your own personal prayers aside from the Agpeya. However it should not replace the Agpeya but complement it. Therefore, you do not practice one without the other.
The important thing is that you need to talk to your spiritual father about this state you are in these days and listen to his guidance. He is the one who can lead you step by step into growing spiritually in your prayers and overcoming this hindrance in your prayer life.