It can happen, you just need to keep in mind what's important

It,s possible to be friends with an ex so long as you can keep in mind what,s important to you. For example, if you,re in a relationship with someone already and your ex starts talking to you and you,ve got some of those lingering feelings for them -- get it straight in your mind whats more important to you: Being with who you,re with now or taking it as a chance to get back together. If you,re cool with sticking to what you already have -- just keep it in mind and set a mental boundary stating to keep things at a FRIEND ONLY state. On the other hand, well -- I,d say it,s about time you confessed or tell your significant other that you need some time to settle straighten things out if you,re having trouble doing it.

Long story short:
If you want to be just friends, keep it in mind and you,ll be fine -- think about the days before you two were together. Don,t go for more or less.