Reducing from 100% to 80% LoE

I am currently working 100% and already expressed my wish to reduce to 80% a few months ago. My boss is against such a model since he considers my position as an entry-level position, in which people only stay 1-3 years. Thus, according to him, it would not make sense to allow us to reduce working weeks. In the rest of the company, people are able to work 70-90% and many staff make use of this option.

The fact that he considers me not as a long term staff and implicitly my work as not very challenging (as someone new could learn it easily after I or someone with my position has left), is highly demotivating for me (especially because I am doing the job since 2 years and still have many situations where I learn new things). Also, the fact that we are not treated equally to the rest of the company feels unfair to me.

Could you help with managing my mind around that and thinking/feeling/acting solution-oriented instead of unmotivated/offended?