I've just timed the song halfway and already give up.
this picture alone describes every single experienced mappers' nightmare: inconsistent bpm in just one small section. Yes, you're mnot wrong, even famous mappers avoid these, pishifat had a quote for it: "It's worse than torture".
With the newbie like you, just map another song is the best option right now. I'm not insulting you, nor saying that I'm suck, this is literally the most impossible task to do in the mapping scene. And if anyone can rank this kind of song (actually some can do that) then they're consider "god" of mapping (I'm not exaggerating, the person can do it actually have a lot of patience, you can try if you want).
So yeah, please just map another song...