I'm looking for a yaoi that has a uke that initiates his love for the seme first, rather than it being vise versa. The uke should have a tsunderish/impulsive personality.

(Also, please do not recommend me Junjou Romantica or Sekai Ichi Hatsukoi, as I have already seen them.)

You could try doing a category search with Yaoi + Tsundere.

The only title I could think that kinda fits your request is Kamatte Kawaii Hito, but it is really short. Oh, and maybe also Koi to Kaerou...? It has been so long since I last read it. I think these both titles have the uke falling in love first, but I don't think/remember they taking the initiative.

In yaoi, the tsundere character usually is the one that falls in love later, trying to hide his feelings/admit he fell in love... it is a bit hard to find tsundere + uke with initiative.