The reason that all the Tab on the net is in F is that the standard key for playing the Girl from Ipanema has always been F--Jobim and Gilberto did record it originally in Db, but that is a key that is highly unpopular with the instrumentalists who typically make up a jazz combo( though not a problem on guitar) and it got moved to F--Jobim even recorded it there when he did his famous duet album with Sinatra--

The reason that you are having a bit of trouble playing the guitar parts is simple--bossa is the most complicated music that there is(at least of the kinds of music that people actually *like*)--it uses samba style polyrhythms(generally four distinct layers, not counting the melody or solo line) and progressive jazz chordal and harmonic rules(which, especially in 60's jazz, meant *no* rules)-up til Bossa came along, music tended toward either rhythmic complexity or harmonic complexity, Jobim and company thought it would be fun to do both--

Bossa is especially hard on the guitarist, who is responsible for playing all the rhythmic layers, as well as the chords, and, often, is expected to sing over the whole mess, as well.   Still, if you get the proper instruction on how to fit it all together, it works, and over time, even becomes as effortless as it sounds--