How do I handle knowing people's lives depend on what I build?

It is settling into you that the bridge, the building, the road you design is not just a technical problem, it is a promise to every stranger who will trust it with their life. That weight is real, and feeling it is what makes you the kind of engineer people should want. Do not try to shake the weight off. Carry it well. It is the thing that will make you check the calculation one more time, question the assumption, refuse the shortcut when refusing is hard. The engineer who feels nothing is far more dangerous than the one who feels the responsibility keenly. Your seriousness is a safeguard. Let it settle into a steadying habit rather than a fear. On your next design, name to yourself the real people who will depend on it, and let that thought guide the care you bring. The weight is not there to burden you. It is there to keep them safe, through you.