What do i do about things my gf and i have told each other?

we broke up last christmas for a month and each had a drunken encounter kiss, her with someone i thought i knew closely and me with someone she thought she knew closely.. we spilled the beans and she is dealing externally alot better than me it seems but im on the edge of electricuting my self in th bath because we have spent so much time telling each other we are our worlds and everything in it now this is not just puppy love as we have been engaged for quiet a while but with my previous drug abuse we have been through heaven and hell but i really need insight on what to do please! i cannot get over the fact that she has "nothing else matters" tattoed on her back which she got for me and had that on her when this bloke asked her for a hug goodbye at a club and slipped a kiss and she kissed him back.. and considering what i did im seriously considering dissapearing into the night or just plain killing myself.. im putting her through hell with my mental anguish!!!!! this is serious :( :(