lapse vs fall back

lapse is a noun but fall back is not a noun.

Word	Noun	Adjective	Verb	Adverb
lapse	Yes	No	Yes	No
fall back	No	No	Yes	No

As verbs, lapse and fall back are synonyms defined as:

Other synonyms of lapse include recidivate, regress, relapse, retrogress.

lapse (noun)	fall back (noun)
a mistake resulting from inattention
a failure to maintain a higher state
a break or intermission in the occurrence of something

lapse (verb)	fall back (verb)
go back to bad behavior	go back to bad behavior
drop to a lower level, as in one's morals or standards	retreat
end, at least for a long time	hang (back) or fall (behind) in movement, progress, development, etc.
for time to move forward	fall backwards and down
pass into a specified state or condition; sink into	move back and away from
let slip	have recourse to