Relationship friction, loss of control, ego-related conflict

Learning an ex is seeing someone else can be a painful experience, especially if the character is still emotionally attached. The amount of conflict this generates for your character will depend on many factors, the most impactful being who the ex is with and where they are seen.

The character running into the ex and their new beau at a favorite restaurant
The character realizing their "replacement" is a friend or family member
Running into one's therapist, pastor, or trusted mentor and their date ... who happens to be the character's ex
Showing up for a team sport and seeing the ex with one's rival
Running into the ex and seeing them entwined with someone they always denied liking, which may become a double blow if the character suspected the partner of cheating in the past
Attending a corporate social event and discovering the boss' significant other is the ex
Seeing the ex at a funeral, being comforted by someone else
The character returning to the old place to retrieve their stuff and being greeted at the door by their replacement
Attending class or a school dance and seeing the ex with a hated rival or enemy

The character saying something they'll regret later
Awkwardness or unease that causes the character to do something embarrassing (spilling a drink, putting on an obvious act as if everything is fine, etc.)
An onset of tears that can't be hidden, leading to public embarrassment
Skipping school or calling in sick to avoid the new couple and getting in trouble for it
Avoiding the ex by canceling plans with a mutual friend and creating tension in that relationship
Old feelings cropping up, causing temporary uncertainty about a current romantic partner
Becoming possessive or needy with one's romantic partner