Existential angst is an essential part of the spiritual journey. It passes of its own accord and there’s not much you can do to hurry the process along. What may help is more context as to exactly what the spiritual process is doing to you, under the hood, so to speak.
There’s this thing scientists call the hedonic treadmill. Basically, no matter what you do in life, win the lottery, get married, lose a friend, your subjective level of happiness returns to the ‘set point’. It doesn’t matter how fast you run to get off the treadmill, you won’t ever attain joy / happiness that lasts longer than it takes to get you back to your set point.
Spirituality… messes with this. Awakening expands your sense of the possible, and most importantly for the purposes of this answer, uncouples your sense of happiness from the things that happen to you in life. No longer is your sense of well-being / joy coming from things you work for and attain in life, it comes from, well, depending on just how enlightened you are, from thinking spiritual thoughts all the way to just plain existing.
In my life, satoris can come out of absolutely nowhere. They’re so frequent that, like a decade ago, I’d long since stopped caring whether I was in one or not, or how to obtain them, or how to hold on to them when they arose. I don’t need pick-me-ups, and getting down is just as interesting to me as being high.
My mood is far more complex than anything that you could represent on a two-dimensional chart. The relative level of joy / depression is just one variable.
This is what the depressive phases of spiritual awakening are ‘designed’ to do. There are two aspects, deadening the parts of ego that make you hold on to your current place on the hedonic treadmill, and giving you the opportunity to find greater depth in whatever it is you’re feeling at the moment.
They also serve another purpose. Most people who awaken spiritually are rather young. They’re not done maturing emotionally yet. In many ways, we can look at the extreme mood cycling of awakening as an extended adolescence.