
It took Dahye a couple of months to realize she likes Yuna. Remember when Yuna and Joy started getting close Dahye felt jealous; and Jooyoung even told Yuna that Dahye had stopped talking about boys the past months. So it took long.
And Yuna even told Joy that she dated Joy to forget Dahye. So in the end, I think you missed some points here.

I understand your point of view but:
1. Yuna can't seem to be sincere, she liked Dahye for 10 years as you've said: why? What's the point of hurting yourself looking at the person you love date men for 10 years? Like she was persuaded that Dahye was straight so at the beginning there was no hope of à romantic relationship so why is yuna sticking by her side? Love is one thing but allowibg yourself to get hurt for 10 years is being a coward.
2. Why didn't she confess, I know she loves her etc. She wanted to stay by her side even though that would lead her to being hurt blah blah but at least she would have been sure of what was happening, could move on, or maybe Dahye could have reconsidered her feelings and realize she loves her back.
This amount of desperation/ seeing her with boys for 10 YEARS like girl do something? Anything?
I can't applause her for waiting for her for 10 years, 10 years of suffering watching her with other men/ not being sincere with her feelings/ not being able to move on... like this is unhealthy... and I don't recommend anyone doing that unless you like to get hurt.
And once she finally made à " progression", when she finally started to try to move on because she was desperate Oh! Dahye is now in love with her, this just seemed like jalousy but not in a romantic way. It was like "hey you took my favorite toy it's mine give it back" when she stayed at Joyoung? (Sorry I don't remember how it is spelled) she showed sign of jalousy and said " Yuna only usually stay with me, I never saw her taking à pic with someone else" like it just seemed like à possessive bestfriend pissed that Yuna could think about something else but her.
+ there is not even sight of a " romantical tension" between yuna and dahye, it just seemed like dahye was scared to loose yuna to someone else because yuna only looked at her, stayed with her, prioritized her and never said no to her for 10 years and once it happened it was shocking because it was the first time it happened, it was unusual and it might have hurt her. No explanation of how it happened was really provided just jalousy and fear of losing her to someone else. Of course Dahye cared for her (the car/ when she was drunk etc) but it was sooooo fast like hey! I am scared to loose you, I am possessive of you cause you my bff so I like you let's date so I make sure you are only mine!
Her jealousy was soooo unhealthy, yuna sticking by dahye's side when thinking she was super straight and getting her feeling hurt everyday for 10 years was soooo unhealthy like it was not even cute...
It was so unhealthy even Dahye's brother tried to help her move on, like OK she chose her path but girl she was going nowhere and stuck miserably in the same position for 10 years. Mm
Now 3. Talking about Joy. Ugh where do I start? Yeah it's great she moved on "easily" and did not seemed too hurt but wth. Ppl are not toys ? Like you don't use them and throw them away when you are done? Like if you can't get over Dahye and you know it, first be sincere with dahye and Joy by telling them what is happening? And at least if you know you can't get het out of you mind like don't date Joy? So you are not hurting her by the way? Like why?
All of this was so unecessary, so much toxic ( you don't use people to forget about others this is dirsrespectful ) toxic behaviour, not being honest and bring a coward, being possessive. Like the only one that had her shit together (Joy) and was doing well was the one that got hurt in the end.
Things went so fast, brievly explained, it doesn't even seem sincere. But they are cute together I guess

I would also like to mentioned that a lot of people dates other people to forget about their exes or unrequited love. And that’s what Yuna did, but of course it didn’t work out for her in the end. Is it technically using a person like a toy? No it’s just a shitty thing called rebound(I don’t like rebounds either don’t worry).
Anyways I’ll also like to mention that Dahye didn’t know she was in love with Yuna, she can’t just immediately realize it out of no where(she had nothing to go off from). Plus she basically confirmed her feelings for Yuna, when she said, “If I was a guy I would also like Yuna, but I am a woman” this happened in her backstory(further confirming that she is a Bisexual in denial, remember it’s not common to come out in Korea). Which is also why Yuna didn’t confess, reason being Dahye is her best friend; she didn’t want to ruin it if she were to get rejected.
Anyways I don’t think they got together from a mere jealousy. As said before Dahye took months to find out what her jealousy was about, she even seeked help from Jooyoung; who she knew was lesbian(because she caught Jooyoung kissing her girlfriend). And so she found out her type of jealousy is not a friendship kind, but a jealous lover kind.
And lastly, there were romantic tensions. In the last scene we see them confessing. Dahye’s super cute flushed face, and Yuna’s excitement. Their hug, their kiss, and lastly the underwear and hotel scene(it was funny and adorable to tease Yuna, but she wasn’t kidding when she said she was gonna wear cute underwear).
I am disapointed, like fr? All of this for you to end up with dahye like bruh, fr? And you ended with her so fast as if it was so easy and all of a sudden she loves you back?
Joy deserved better than being played like that.
All of this because you can't be sincère??? Uuhggg I am pissed