To buy you time I would say to your mum that you appreciate the effort she is going to with the study but as she says it is your arrangement. You do not feel the current arrangement is helping you to make the truth your own as you feel you are being prerssured and don't have time to take things in properly. This may be ok in the short term but it will not help you long term. Certainbly having others set spiritual goals for you means you are not having an opportunity to build a personal relationship with God or grow to maturity.
You want to take a break form this arrangement to give you time for personal study and reflection so you can build your own foundation. Then you can tell her you will restart the study when you have something to offer rather than just learning lessons by rote.
Do not allow yourself to get into a debate deliver the statement and walk away. If she asks to discuss it just say you have taken the matter in prayer to Jehovah and you are confident that this is the best course.
In the mean time try to find a part tinme job to give you some financial independence and the opportunity to have work commitments that occasionally encroach on meeting attendance.