elementary particles (e.g. protons)
Protons aren't elementary particles, they're made of partons (quarks and gluons) in "soup".
Below, $\lambda$ is the wavelength corresponding to the energy of the interaction via the usual de Broglie relation and $r_p$ is the radius of the proton.
At low energy with $\lambda >> r_p$ the interactions are just like you describe, the protons are point-like and postively charged and repel each other.
At mid-range energy with $\lambda \approx r_p$ the proton is no longer point-like but behaves as a uniform charged body.
At high energy with $\lambda < r_p$ the spatial resolution is precise enough to involve interactions between individual quarks and the proton behaves as a bundle of 3 quarks (uud).
At very high energy with $\lambda << r_p$ such as those currently reached by the LHC the 'true' nature of the proton is revealed as containing an ever changing, bubbling soup of quarks and gluons popping in and out of existence (even other quarks besides the u and d that are "normally" present, which is how we can produce B-mesons in the LHCb detector even though at first glance you'd think there are no b quarks available: a certain fraction of the time a b quark from the quark-gluon soup within one proton interacts with a quark or gluon from the other proton via an exchanged gluon and then goes flying off before hadronising into a jet of hadrons, including B-mesons).
So basically, at low energies protons do behave just like positively charged point-like particles and bounce off one another, but at high enough energies the wavelengths of the exchanged bosons become small enough to single out the individual constituent quarks and gluons within the protons and they therefore interact individually.
why do they break up into dozens of other particles
This is the process I mentioned above, "hadronisation". It's a consequence of QCD confinement. Wikipedia will give you a more detailed explanation if you want one, but a qualitative understanding can be gained by knowing that coloured particles (i.e. quarks and gluons) cannot exist on their own and have to pair-up (red-antired, blue-antiblue or green-antigreen), or trio-up (red-green-blue or antired-antigreen-antiblue) to become "colourless" mesons or baryons, respectively. Mesons and baryons are each types of hadrons.
The coloured partons produced by high energy proton collisions emit gluons which pair-produce quark-antiquark pairs by the hundreds or even thousands. These can all then cluster together and become colourless hadrons. You can imagine how complicated and chaotic this process is, which is one reason it's very hard to model.