Once a person has been betrayed in a relationship, they are very likely to develop deep trust issues that may take a very long time to get rid of. They may become paranoid and obsessive in their new relationship.
It may be cute at first, but it soon becomes very tiring and annoying when it feels like someone else is watching their every move 24/7. Luckily, there are a few things you could do if you find yourself in one of these kinds of relationships.
Certainly, the change isn't going to happen in a day. Your partner is not going to attend therapy and emerge a totally changed man or woman. Like all good things, it will take time, but there are high chances that it will get better.
If you are dating someone who consistently brings up how their former partner cheated on them and broke their trust, you might want to ask why. The answer may seem obvious, but sometimes, the answer may be deeper than it seems.