@ericstene I'd like to extend my Welcome to Mayo Clinic Connect. After 50 years of sufferings from anxiety you are being treated for the first time. As you, "make progress against the anxiety, [you] have gained some mild depression." For the last few weeks in the morning your, "mind [is] busy with positive plans" but shortly after, "like a switch was thrown and [you are] immediately back [to] anxious, a bit depressed state for the rest of the day."
50 years is a long time to have anxiety. Since you have been anxious for 50 years the habit of being anxious is well formed. It is possible to build new habits but it takes time. We build new habits by changing our thoughts and our behavior. Literally our thoughts and behaviors change the structures of our brains via neuropathways.
Would you be open to asking your therapist for some psychoeducational materials related to neuroplasticity?