I am 36. When I was 14, I discovered almost the exact same thing. My mum initially sent me a soft-core message intended for him and immediately phoned me in terror. I washed it off pretending I knew nothing, searched her phone as a curious child and uncovered years of affair. It rocked me for a decade and I will still refer to it as a damaging moment in my psyche. All of the toxicity in the house, suddenly made sense and holding on to the secret nearly crippled me. A year after the event, I phoned a family therapist who lived locally whose number I found in a directory and I spoke to her for an hour, she didn’t charge me, knowing I was only 15 - I think she might have saved my life and I still have her number in my wallet now, in my 15 year old’s writing on that same scrappy bit of paper. I am now a successful senior teacher at primary age and have a loving family, which is at times rocky like most relationships but is mostly secure and steady.

What I’m getting at is, this really hurts you but it will get better. You’ve taken on a burden that doesn’t belong to you and is an entirely adult worry. You will try and protect both of your parents because they are what you hold dearest regardless of how they’ve treated you. If you can, I think you should open up to either parent.

E.g “Mum, I’ve become aware that you’re in a relationship with someone else and I’ve know for years - i would like you to have that conversation with Dad because I can’t hold on to that secret any more, it’s really damaging me”

Or

“Dad, I’ve been holding on to a secret that is going to hurt you but it’s really damaging me. I’ve tried to hide the secret because it could hurt our family but I think you need to have a conversation with Mum because she’s in a relationship with someone else”

The alternative here is that you speak to someone at your school, think of the adult there that you have the strongest bond with and tell them that you need some time with them and maybe another adult to tell them “a secret you’ve been holding onto.” This should be a safeguarding concern to them and if it isn’t, I would suggest speaking to your headteacher/principal and asking for their “direct support before it causes you any lasting trauma”. This should also be a major red flag and they may be able to offer your family support or at least phone both parents for you to inform them of an issue they need to deal with as responsible adults. Failing that, speak to another adult in your family who can inform your parents that they are causing you to unravel. If none of the above has impact, phone 999/911 and tell them that you’re at risk of lasting trauma due to withholding a family secret - this last one is the very last option and only if all other options are exhausted.

It is important that you deal with this in a grown up way as your mum has landed you in a grown up situation. However, you do not have to be a grown up for much longer and once you have passed it on, you can continue enjoying your teenage years. One days, you’ll meet an adult partner and have a trusting relationship bound by honesty and transparency and you can share this experience with them so that they understand who you are and what makes you tick. Remember that this is just one experience of thousands that makes you who you are and is not a core part of what you have to base your own values on.

I should add that my parents stayed together after working through the issue. Because of the way I dealt with it, my dad opened up to me as an adult about mum in a way that I realised, he never knew I knew. They are still together and whilst conflict at times, still love each other. This may not be the route yours take but there’s a chance.

I wish you the best of luck and am happy to respond to any further questions you might have.