how does Lucie cope with her worries and fears for her husband?

Lucie is actually quite strong under the circumstances....... she follows her husband to France, intercedes with Mrs Defarge (to no avail), and methodically stands outside her husbands window daily to give him hope.

After Darnay's release, she falls apart a bit. With him there, she can allow her fears and emotions to spill over; she was strong while she was alone and waiting, but she allowed him to be the strong one in the brief interlude between arrests.