Feelings are there, it's just our awareness of them that changes. Trauma causes intense emotions, more intense than we can cope with. The brain does a pretty smart thing by teaching itself: emotions need to be silenced. Emotions might cause us to lose control if they're too intense, or they might even cause someone to hurt us. So the brain has built in a protection system - numb the feelings, stay in control.
Self-harm causes physical pain, but the need to self-harm is all those silenced emotions trying to find a way to express themselves. Self-harm, pretty much by definition, is a way of hurting in a way that you have control over.
The degree of control that we have over our self-harm wanes. Like any addiction, the brain's need to feel that release starts to overtake our ability to rationally assess how much control we actually have. Scratching becomes cutting. Drinking becomes alcoholism.
To feel emotions in the 'real' sense, the way that healthy non-traumatised people feel them, you're going to have to prepare yourself for the wave to hit. Because it will, eventually. Reteaching your brain that emotions don't need to be silenced anymore is going to release all the stuff that your brain has protected you from by numbing it down. There are skills you can learn, DBT and mindfulness are good examples, that help us teach our brain not just that it's okay to feel emotions now, but also how to cope with them when they hit.
All that is great information to have. The catch though, the bit that you don't want to hear - you have to stop self-harming to feel. Doesn't happen straight away. But as long as you keep self-harming, your brain has a (ultimately destructive) alternative in place, which it will cling to. Because you control it.
At some point, you just need to stop. That's the deal. There will be void, numbness, emotional silence. But eventually, your brain will realise that there's no more self-harm-sedative coming, no emotional-xanax, and they come.
Big scary process. Lots of painful beliefs, habits, thoughts, that all need to be confronted and upended. But it does happen. Be gentle with yourself - allowing yourself to feel isn't all bad. Yes, you will start feeling the sadness, but you will also start feeling the joy.