Each stanza of "Celeste" starts with the same fear: "If I lose..." Sight, sanity, grip, direction, and finally life. The speaker is talking to someone close, and every time they ask for something in return. At first the requests are about staying steady: see the world for me, be you for me, keep your hold on me. These are the things you'd ask of someone who might have to look after you.
The later stanzas get bigger and sadder. The speaker asks the other person to go and save the world, then come back, and later to rule it. Those lines feel half serious, half like a gift of everything the speaker has. The last stanza surprises us. Instead of asking the listener to hold on, the speaker lets them go: live your own life, don't look back. Then comes one small wish, "Please remember me."
The mood is tender and quietly sad, close to a goodbye, though the short lines keep it calm and simple.