Can you accept your own weakness, asking God’s Help and make your fast say to 3pm for a week and then 3.30pm for another week or so, or 4pm - and very slowly adjust to a fast for 24 hours?
The Good Lord is not only in ‘our’ achievements which are His anyway since nothing good comes about without His Grace and The Holy Spirit - but He is also in good and ardent desires and indeed how we approach Him in our weaknesses and with humility. In otherwords your desire alone to fast for Him is a good act through Grace and The Holy Spirit and accepting your weakness and working with it hoping to move beyond it with His Grace is a humble act - and a good act which only can be achieved with His Grace and The Holy Spirit also.
I dont think personally that a fast for say 8 hours is any less meritorious than a fast for 24 hours - some do not find fasting all that difficult while others find it near on impossible and can only fast with great effort and then not for long periods. But always humility is the foundation of all virtue and it is never what I can do for God but what God can do with me if I only let Him. But if you really would like to fast for 24 hours, then taking things slowly, humbly, and gradually increasing the time span may work for you. We are all different even in our biological makeup.
Do you have a spiritual director? - if so be guided by him or her and discuss the matter.
TS