Affirmations can have various forms. You create such an affirmation that feels so natural to you that it literally transforms your subconsciousness. You say something like this: "I've totally overcame my doomscrolling addiction." But this indicates the following:
1. You are struggling with doomscrolling.
2. You are addicted to doomscrolling.
3. You try to overcome it. But for how long? One day? One week? One month? Years?
This affirmation focuses on the problem you try to get rid of in the first place. Focusing on this as an addiction might result in you being in the state of struggling and fighting against it. But you clearly do not want to be struggling with this. You clearly want this problem to be erased in a way that it would look like it never existed in the first place. So, instead of focusing on this struggle, you might focus on the positive thing you actually want to have. Maybe it is using your own time efficiently? And the affirmation would be as simple as that if that's the case: "I am using my time efficiently in the best way possible to me."
But even this affirmation can be too broad. What would it mean to use your time efficiently? This is where you would analyze why doomscrolling is the problem for you in the first place. What does it result in? Do you spend less time developing yourself? Does it harm your professional life, relationship? Let us assume (just an example, your real situation might be different) this doomscrolling results in you showing less attention to your loved one. If this is the case, then the affirmation can be this: "I build a better relationship with my loved one in the best way possible for both of us."
In other words, the best way to solve some problem is to indicate what this problem does to you, how it makes your life have less quality. And then after indicating those things you can just work towards improving those areas of your life. If the indicated thing is the consequence of your doomscrolling, then improving that indicated part you will automatically doomscroll less or stop it completely eventually.