I am using the actor/actress real name to make it less confusing. Ki Yong is the office worker male lead Jang Hyuk. Soo Bin is the female lead that is a nurse.
She and Ki Yong weren't ready for the baby which was why they must have discussed about this abortion together. Before she ask ki yong to go to his car, she saw how stressed he is and how fed up he was with all of this which further encourage her to go through the abortion (even though she actually didn't want to kill the baby and want to keep it instead.) However, Ki Yong never discourage her from abortion and he even thought the baby was a liability after seeing all the mother and children in the clinic acting in a way that is not like a happy family way.
After the abortion, she actually cried on the bed once Ki Yong left for his work (even though soo bin told him to go to work, she actually wanted him to stay with her and take care of her). She still regrets rejecting the kid which was why when she told ki yong about how he abandoned his kid, she thought he would feel remorseful about the act but he blamed it back on her again. before that big argument in the car, she was trying to reconnect and make the relationship work out (which it didn't as he kept feeling pessimistic over everything and even starts to call out Krystal's name + the gold burger set which Krystal likes to eat) . She then smoke a cigarettes which I think is evidence of her abortion cause you wouldn't do that to your baby.
There is some theories out there about her still being pregnant because the airport scene shows her handing her luggage bag to Lee Jang Hyuk and having a hard time standing up but I think she could also be in a trance about her past relationship and also not looking forward to this trip.
You can say that Ki Yong never actually physically cheated on Soo Bin but he was already emotionally cheating on her when he neglected her to that degree. He broke his promises of fixing the lightbulb, not coming home when he should have and he is not even spending time with her. He always focus on himself and his job without even considering what she was doing, how she was feeling and how did her job went down. When she started talking about her patient (Lee Jang Hyuk), he didn't even bother to listen and is laser focus on his "revenge" to Krystal.
So when Soo Bin is the only one that is actually giving in this relationship and is trying to give love to Ki Yong, Ki yong didn't respect that and disregard that further. his heart was already stolen by Krystal to the point where even the security guard see how infatuated he become to Krystal (ki yong was denying his love to krystal but the security guard sees all with his CCTV set up). Thus, the D-day came when he called Soo Bin a different name, ordered a burger set for no reason (Soo Bin's character seems to like rice), didn't notice her perm, didn't even tolerate the idea of eating the macaroon when she offered and him blaming her forced her to finally made the decision in finding comfort with someone else (whom share the same name and job). By that point, I wouldn't consider her physically cheating on him cause they broke up and on the next day, he also proceed to hook up with Krystal. (though they never say a word about it which is why at the end, Kiyong thought he had a chance but he lost her already)
She is sad at the fact that she gave Ki Yong her love but never received anything back so this time she wants to be the one that get showered with love from a person (coincidentally has similar names and job). Even in the ending scene, she was still thinking about the past and she "scolded" LJH for not carrying her luggage bag. The matching shoes was never her intention with LJH (only for the 50% discount) but in the past, she willingly give the same matching shoes to Ki Yong as her boyfriend. This proves that Soo Bin may not actually like the fat Lee Jang Hyuk because she just want to receive love and her act of getting close to him at the start were mostly her pregnancy urges as well as her duty as a nurse. She just wants a replacement of his to fill her missing part.
