Why do I feel like this?

I was feeling really down like a year ago I think it was the lowest point I ever was. I struggled with sh back then but I´ve been clean since then. I never thought I was addicted to it but I thought about it every time I felt upset or something happened so I guess I kinda am. I never was diagnosed with depression so I always think my feeling are invalid because why do I feel like this if I´m not depressed. I am diagnosed with social anxiety and selective mutism which makes me struggle a lot in my daily life. Lately I have been feeling really bad when I´m alone and have time to think and am not distracting myself with something. I have been thinking a lot about hurting myself again, I didn´t do it tough. I just don´t know why I feel like this again and I just wanna have happy thoughts when I´m alone. No one needs to read this I just wanted to let this go and I´m sorry if I made any mistaked with grammar and stuff, English is not my first language