I'm scared my partner will leave me if I show how I really feel

Every time I'm honest about how anxious or low I'm feeling, a voice in my head says they'll get tired of this and leave. So I hide the bad days, which makes me feel more alone, which makes the anxiety worse. They've never given me a reason to think they'd go. But I've been abandoned before and my nervous system remembers. I hate that past relationships are running the show in this one. How do you stop waiting for someone to leave?