for me, this usually happens at night. And i just exuse it as "getting in my head" but in reality, it is my depression. What i do is write. Just organize my thoughts. But sometimes it just helps to stimulate your senses. like going for a walk to clear your head or eat something. and if it is intrusive thoughts, a freezing cold shower. Because the shock will help.

this is the reason i cant fall asleep without some sore of background noise. i have ADHD and depression, so my mind tends to wander a lot, and when it does it's usually negative. the only way i've found to combat this is to listen to podcasts or creepypastas while i fall asleep, just to keep my mind on a story or a conversation. i've also found that the headspace podcasts work well for me, but i haven't used them since they went behind a paywall.

Yes when im not doing anything and I just think about stuff like that I go into a dark place! Buy I have found constantly keeping myself busy really really helps, even my therapist thinks so! Like doing chores, watching a show, exc.