Zhisu/Lewis stayed with Jiang even though he knows he's cheating on him because he loves him so much that he will endure the pain emotionally(Jiang cheating), mentally(anxiety of dying) and physically(Illness) he accepted all Jiang's flaws and loved him unconditionally. For him it's fine to stay by his side and wait for his life to end until Jiang physically hurt him. That time he was broken hearted because Jiang hit him, he still loves him but it's too much to bear for him to stay so he decided to left Jiang. Zhisu ended up with Dr. Ai and he accompany him to the end of the road. I believe Zhisu didn't fell in love with the doctor but he wanted to reciprocate Dr. Ai's love by giving himself to him(physically) and agreeing to stay by his side until the day he died. Notice whenever Dr. Ai tells Zhisu tells that he loves him Zhisu never say he love him too. Zhisu tells Dr. Ai if ever he gets to live another life he will go to him and only for him, that means he wished that he met Dr. Ai instead of Jiang so that he could love a person that will care for him. He wished he wasn't tied to the love he and Jiang has. I believe if you undergo a break up you might not be able to move on in just a couple of weeks or so that's why I believe Zhisu hasn't moved on until the day he died but have the peace because Dr. Ai was with him until the end. I really love the story, I wanted to to cry but I can't something must be wrong with me. T-T
10 years where I loved you the most