Loss of billable hours

Background: Part of my business revolves around virtual assistant work. With one client, I have worked 20 hours per month, for the past 4 months, doing social media management for her marketing clients. I have come to rely on this steady income.

My client has since ended her maternity leave and is working to scale her business. This week she has hired on another virtual assistant and has cut my work hours down to 10 hours per month. The other 10 hours is now being given to the newly hired virtual assistant.

I feel that my past work and skills are undervalued and unappreciated. My initial instincts are to be more strict on project turn around timelines, since this client is no longer at a premium level. I feel this action would be more of an act of lashing out rather than a sound client relationship decision.

My question is: how can I move past this feeling of undervalued and move forward with this client relationship?