sickeningly sweet

It all starts out sickeningly sweet.

If a situation is 'sickeningly sweet' it is so sentimental or romantic that it is difficult to watch.   Notice the following:

1. They are sickeningly sweet together.
   
2. Her love songs are so sickeningly sweet that I almost can't stand to listen to them.
   

get serious

We started to date and we got pretty serious

If a relationship 'gets serious' it becomes more intense and looks like there is a future for long-term commitment. Notice the following:

1. How long before you two got really serious?
   
2. I think this relationship is going to get serious.
   

get rid of or ex

He was trying to get rid of an ex who kept bothering him.

If you 'get rid of' something you are finished with it and you want it out of your life.  When we are talking about things it has a similar meaning to 'throw away.'  An 'ex' is a person that you have dated or been married to in the past.  Notice the following:

1. I am getting rid of a bunch of clothes.
   
2. She is still good friends with her ex.
   

writing back and forth

We were writing back and forth, telling each other how excited we were.

To 'write back and forth' is to communicate through letters, e-mail or another form of written speech.  Notice the following:

1. We wrote back and forth for a long time before we met.
   
2. She communicates mostly through writing back and forth.
   

add insult to injury

He added insult to the injury by saying we were always just friends.

To 'add insult to injury' is to make a situation worse by making an additional horrible comment or action.  Notice the following:

1. To add insult to injury he started dating her best friend.
   
2. To add insult to injury the stupid thing broke two weeks after I bought it.