A resistive voltage divider won't work for the reason you already worked out, the load (the RC project) is effectively across the bottom resistor making it's value appear to be much lower.
The simplest solution here is to use a linear 3.3V regulator. They cost almost nothing and take in anything from about 4V to 30V on their input pins and give a stable 3.3V on the output pin. The downside is they are inefficient, particularly when dropping a higher voltage. The more they drop the hotter they get.
A better option is a regulated SMPS supply. They cost a bit more but run cool. If you search for regulated SMPS you can find many that give a fixed 3.3V (it's an industry standard voltage) or have an adjustable output you can set to anything lower than the input voltage. Example:
https://www.ebay.co.uk/itm/DC-4V-38...469670&hash=item1c89dda845:g:GqEAAOSwVpVbIBvw
Brian.