I recommend lowering the pH at 3.2 with a buffer using ascorbic acid/ascorbate system, and protect from light; Triprolidine is light sensitive and is pH dependent.

See and adapt the information in this article;

Article Stability and Stabilization of Ascorbic Acid

Triprolidine HCl is sensitive to light and high pH. I recommend you to use an acidic pH with appropriate buffer capacity or formulate your system in suspension form. You can also exploit microencapsulation to protect the active ingredients from deleterious environmental factors.

You can also add sugar to your solution if it would be possible.