You have established that 45 minutes is stretching your 'concentration span' to its limit. Different people have different concentration spans, so accept your limits and use them to your advantage.

You need to 'stay fresh' by doing short stretches of 'intensive learning' interspersed by recreational playing - or rest.

The learning process can be likened to microwave cooking - the cooking carries on after the food is taken out of the microwave (the 'standing time'). In the same way, the learning process continues after you stop practising!

If I want to learn something 'permanently', I'll have a recording of it playing in the car whilst driving, and I'll have short sessions at it when I can. Reading through words last thing at night before going to sleep, and reading them again when I wake up seems to work best, and a similar approach applies to difficult fingering for a tune or accompaniment.

'Constant dripping wears away a stone' is what I used to preach to my students... the spacing between the drips is the clever part...