Telling a partner about an STI feels daunting, but it is important for their health and is the responsible thing to do. Choose a private, calm moment, be honest, and share accurate facts: which infection it is, that it is common, and that it is either curable or manageable. Knowing the facts often reduces fear and stigma.
For current partners, the conversation lets them get tested and treated so you don't pass the infection back and forth. For past partners, many sexual-health clinics offer confidential 'partner notification' services that can contact them on your behalf without naming you, which can make the process much easier.
A licensed doctor or sexual-health service can give you the right information to share, advise on which partners need notifying and over what time period, and support you through it. Treating partners together is often necessary to fully clear curable infections and prevent reinfection.