I don’t know what to do with myself

I’m 23M. I recently ended a 4 year relationship that’s been taking a toll on me. It was a mutual breakup, and there’s no hard feelings. I would often feel so rejected and I fell apart as I watched them slowly lose feelings for me. With that, my self esteem died along with my relationship and I forgot how to be single and alone. For context, I met this girl at my job after just one month of leaving my previous relationship before her which lasted about 1 1/2 years. So I’ve been in a relationship for my entire adult life. I have a friend group whom I love but they are so busy with life and when I do see them I feel happy with myself. But when I’m alone with no plans I spiral and cry and escape my problems by doom scrolling tiktok and Instagram reels and YouTube videos all day. I always get high now. I’m doing everything just to avoid this awful feeling. Can someone give me advice? How did you cope with being alone? And how did you handle those episodes where you are laying in bed at night and they are all you can think about?