practically, do I do? How do I do it?
Changing beliefs is tricky - no doubt. And they don't yield to just argument along (mostly) Logic can indicate a path, but won't, by itself as you say, do the trick most times. (It is convenient when it does...)
One thing you can try is to identify things you DO based on the belief you want to change (a lot of people with developmental trauma/neglect have problems ... BIG problems with self care) and then change those habits. Usually feelings come up when we try to change our habits and actions.
Another thing to try is to imagine yourself talking to that three year old self and listening to what she says about herself and letting her EXPRESS that through your body. Stay present - conscious and a bit apart from her, but pay attention in a friendly sort of way. You can be reassuring/nurturing if you like but you don't HAVE to do that.
Another thing to try is to rewrite your early childhood how you would rather it have gone - live it out (re-live it out) in your imagination. If emotions come up ... see above.
Beliefs are HABITS at the end of the day. The only way to get them to shift is to consciously change them and retrain yourself. They tend to have ... roots? Shoots? out into lots of things that you might not initially expect. That's part of the process of discovery.
One thing my T has people do at the beginning is identify a belief - and wear a rubber band on your wrist - every time you catch yourself thinking that thing, snap yourself with the rubber band AND THEN (this is the important bit) repeat what you want to replace that belief with ten times with feeling. Sounds hokey but it makes a difference.
Another thing to do is to write some goals for yourself (with your T or here) in positive language. What do you want your life To BE in concrete terms - how do you want it to feel? Smell? Sound like? What emotions? What do you want other people to say? How they should respond to you. Creating these things is often a process itself. (Stuff comes up... I start "I will be..." and T says, "No say it in the present tense." OK, "I am .... " and I get a few words in and cry. Keep going. Let the emotion work through. Keep at it until you have something that is really positive and true and you can say without a lot of old emotion coming up.... AND you can really
feel in an imaginary way the positive future you are aiming for.
Are these suggestions more along the lines of what you were looking for?
Here is the thing, I have NO major trauma. Zip. None. Super nice life. I have ONLY emotional neglect - but the emotional neglect was pretty comprehensive. I still ended up with structural dissociation. Plus, let me say this again - things seem life threatening for the point of view of a small child that DON"T count as Criterion A traumas according the DSM. So much the worse for the DSM, I say. For example; Here is the huge formative event in my early life. We moved. I had a regular babysitter who I had bonded to as my primary caretaker. We got to our new house on my fourth birthday. I was desolate as I came to understand I had lost her forever (no phone calls, not even a picture - she was just the sitter, right?) And No One Cared. They just ignored me and my pain. My little world had totally and utterly come apart at the seams and .... it didn't count for beans. It was just annoying that I was crying when I should be having fun. It took me a LONG time (ten years with my T?) to get that memory back. But I still start to tear up when I recall it. I split then I think. And didn't attach (really) to anything except animals for many many years. I don't know if that is on topic, but... lots of things came from that...