I'm currently engaged and have a flat with my fiancee, but I keep having thoughts and dreams about an ex (who was my first love) from years ago. How can i stop this?

This can sometimes happen when you're on the verge of making a big decision, like marriage. Sometimes your mind can have second-thoughts because it's not an easy decision to take back once it's made, so you may start over analyzing the situation which can sometimes manifest as thinking about other partners. If that doesn't resonate with you, reflect on you and your fiance's relationship, are there things you need to talk more openly about, are you harnessing any regret, hostility or discontent about things they've done? It's easy to think that previous partners and other relationships are 'appealing' but remember they aren't there going through the daily motions with you, your fiance is. The grass is not always greener elsewhere.