If you know the answer, please tell me.

Hey guys. So I'm going to try and make this as short as possible. So, I'm assuming I have some sort of depression (I'm assuming because I've never been diagnosed). Like, sometimes it's triggered, but other times, I get sad out of nowhere. I drink when I'm sad, I slit my arms, stomach and am contemplating my legs when I'm sad. But the main focus of this is that when I'm sad, I feel so worthless, unloved, unneeded and I need a validation for my existence as a whole. It gets so bad that I reach out to people (mainly my exboyfriend) who serve as toxicity anyways. For example, I constantly check in on him, like just making sure he's okay while I'm on the other end...dying. And sometimes, he doesn't even respond. I think it's because I'm stupid, I was vulnerable and made a mistake. But why do I make it a continuous cycle? Aren't we supposed to learn from our mistakes? Why can't I learn to value myself? Why can't I learn to not treat others better than I treat myself? Why do I keep doing this to myself? Why can't I leave the past in the past and learn to love myself fully? Why does harming myself equal to the betterment of my emotional state? Why do I drink my problems away instead of facing them head on when their bound to do a complete 180, and knock me on my ass again? Why do I keep doing this to myself?