The Previous answer spells out the grammatical equivalent of what is being stated: rather "mashal" is indicating who has rule over you.
From Keil and Delitzsch's Commentary:
"Why art thou wroth, and why is thy countenance fallen?" The answer to
this is given in the further question, "Is there not, if thou art
good, a lifting up" (sc., of the countenance)? It is evident from the
context, and the antithesis of falling and lifting up (נפל and נשׂא),
that פּנים must be supplied after שׂאת. By this God gave him to
understand that his look was indicative of evil thoughts and
intentions; for the lifting up of the countenance, i.e., a free, open
look, is the mark of a good conscience (Job 11:15). "But if thou art
not good, sin lieth before the door, and its desire is to thee
(directed towards thee); but thou shouldst rule over it." The fem.
חטּאת is construed as a masculine, because, with evident allusion to
the serpent, sin is personified as a wild beast, lurking at the door
of the human heart, and eagerly desiring to devour his soul (1 Peter
5:8). היטיב, to make good, signifies here not good action, the
performance of good in work and deed, but making the disposition good,
i.e., directing the heart to what is good. Cain is to rule over the
sin which is greedily desiring him, by giving up his wrath, not indeed
that sin may cease to lurk for him, but that the lurking evil foe may
obtain no entrance into his heart. There is no need to regard the
sentence as interrogative, "Wilt thou, indeed, be able to rule over
it?" (Ewald), nor to deny the allusion in בּו to the lurking sin, as
Delitzsch does. The words do not command the suppression of an inward
temptation, but resistance to the power of evil as pressing from
without, by hearkening to the word which God addressed to Cain in
person, and addresses to us through the Scriptures.
What is expressed is by his anger, he is allowing evil(Satan) to rule over him, whereas by restraining his impulse, he continues to remain in fellowship with God. The individual bears the consequence for his actions, as the previous author states, but it is "who' rules over you which determines your "mashal".