You begin by noticing it. You just did. That's the first and hardest step.
Every time you catch the thought — "I don't deserve this" — practice the counter-thought: "I deserve to be healthy by whatever means work for my body." You won't believe it at first. That's fine. Neuroplasticity doesn't require belief, just repetition.
I also recommend a practice I use with clients: write a letter to your former self. Not to say goodbye, but to say "I see you. You were doing your best. And you deserved help." Because the imposter syndrome is partly about loyalty to who you were — like getting better is somehow a betrayal of the person who struggled.
It's not. It's honoring them.