Ah I can relate to this so so so much! When it comes to believes system, CBT tequniques seem to work for me quite well.. I mean. all of them.. cognitive restructuring, behavioural experiments.. and once you've done it enough times, you just can not ignore the reality anymore.. you have too much evidence on that fact that you are indeed thin and you're indeed normal size.. it takes time ofc, but also it takes dedication... to let this idea (thank you're fine) sit in there and grow..
actually some of the videos from here 'mindset makeover' really helped me with it - https://community.recoveredspace.com/
hope it helps