Quite a few good tips have been mentioned already:

• proper sleep schedule (sunlight early and during day, litlle light in evening / at night helps)
• regular movement / sports
• nutrition (less sugar)

The saying "a healthy mind resides in a healthy body" has some truth to it.

But one thing I wanna mention is journaling:

One of the benefits of psychotherapy is that you have to articulate your thoughts. Turns out articulation via writing gets you quite far and is already a lot better than spinning thoughts in your head only.

Literally just write what comes to mind. Or ask yourself what was good or bad about "yesterday". Try writing on most days, but don't force yourself to a specific amount. Try sticking it out for some weeks until it becomes normal to write. Over time you might see certain patterns, topics that come up again and again. This can also serve as a possible base for later talks with your therapist.

Nevertheless, I still wanna encourage you to try therapy, as having a therapist ask you questions can get you better out of your own thinking patterns (kinda hard if you try it with just your own, single mind).

Theraphy can also function as a catalyst, to get you over the worst part and give you a new perspective, which you then can follow more and more by yourself if so desired. But the answer to that will come to you when the time is right.

Take care, it will get better eventually! Just may take a while, some external help and some patience with yourself.