Seme sees uke with another guy in a restaurant. Uke had broken up with him because he was getting married. Seme becomes friends with uke and falls for him. Towards the end, uke and seme send the dude a postcard of them both. Please helppp
Not very sure but maybe this: http://www.mangago.me/read-manga/tonari_no_usotsuki_suzaka_shina/
I don’t remember much but I remember the uke and seme competing to pick up girls. One day they couldn’t pick up any girls and they decide to sleep together. Another part is near the end when the uke decides to go out and sleep with this random dude to prove a point I think. The seme finds him the park after the uke does that and gets mad and leaves. Please help me!
I think it's "Fuck Buddy" http://www.mangago.me/read-manga/fuck_buddy/

I don’t remember much about it but the male lead knows the main girl was not the real soul of the body she was in? He didn’t care and even got a bracelet to make sure her soul didn’t leave. Please help! :(
I think its https://www.mangago.me/read-manga/grand_duke_of_the_north/
Thank you so much!! Based on the title and cover, I think it is too, but I will check it out. <333333