How can I attempt to inhibit the hydrolysis/degeneration of aspartame?
Aspartame is hydrolysed in both acid and alkaline solution with the methyl ester being the first site hydrolysed, so control of the pH of the aqueous solution is important. Addition of buffer solutions would enable control of the pH, keeping it close to neutrality.

https://www.chemguide.co.uk/physical/acidbaseeqia/buffers.html