my sim is in a relationship but things started to go down hill..

@Lashadae 

Try using romantic social with the two sims.  When the sims have already had a good romantic relationship, you can recover from going negative a lot faster than building it in the 1st place.  

You'll probably have one sim or the other reject some of the socials at first but keep at it and the relationship will eventually recover.

Hope this helps.