One thing I learned from therapy in the last months is the well-known "What resists, persists". It took time to realize that many negative thoughts and emotions that come up or rise unconsciously are nothing more than an old version of myself that doesn't have control over me anymore unless I decide he does. And that old version is not your enemy, it's simply a past you that went through stuff, that subconsciously stuck. And the best part is when you realize that the inner voice is not the real, present, you.
Seriously, be aware enough and one will realize that most times what comes up are not even things that we consciously decide to think of, or feel. And that's when release happens. It might sound controversial but therapy, and more esoteric stuff like the Power of Now by Eckhart Tolle, or the Law of Assumption (yes I believe in manifesting, trust me it's real), and even faith in God, helped me realize that I am the one to decide what will stay in my mind and what will leave. And what I want to leave my mind, I observe it, I feel what it has to say, and then I let it go because it doesn't help in any way to dwell on it.
It's the conscious thoughts that you want to go after and focus on. As my therapist said in our last session "We are who we think we are".