&gt; Is there any way to make this situation better?  

Sometimes, in an attempt to help yourself feel better, you burn bridges with other people.  

Hooking up with S so soon after breaking up with R broke his heart.  
Telling S about R's admission to the mental hospital broke R's trust.  
Sleeping with S on the day of R's birthday humiliated S.  

These actions don't make you a terrible person.  

**But you have to be honest with yourself and understand that these actions poisoned your chances with R.**  

Now you want to have your cake and eat it too.  

You want to take a break from your boyfriend, date the asshole, and then go back to your boyfriend. But your boyfriend is hurting because of your actions. Your boyfriend is made vulnerable because you couldn't keep your mouth closed. Now he's the laughing stock of your group. R's girlfriend dumped him, immediately slept with S, then got kicked to the curb by S, and R is the chump that's taking you back. People will inevitably laugh at how pathetic that sounds.  

If you want to do the mature thing: you need to break up with R, because he needs distance from you in order to heal. Stop being selfish. Stop using people to make yourself feel better. Understand that even though you didn't mean to do it, you have become an extremely negative aspect of R's life. Do the right thing and let him find someone he truly deserves.