In my life there have been many moments, where I was honest with someone close to me, told them the truth about something, and later, wished I'd kept my mouth shut.....there have also been times I was glad I was truthful with someone...
Hindsight is 20/20, but seeing what you need to do now isn't so clear. It's hard to give advice on this one, because you don't divulge what the big secret thing you have to tell your mom is....
Your obscure description of how you assume your mom will react, says this is something that takes the weight off of you and puts it on her...so, you are feeling you'd be selfish to tell her, huh? Without knowing what the secret is, it's all but impossible to give advice, but being a mom myself, I'll try my best...
You can't have an authentic relationship with someone extremely close to you, unless you are truthful with them about the big stuff...whatever that may be. I like honesty the best; lies mess with my head.
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My candle burns at both ends, it will not last the night--but oh my foes and oh my friends, it gives a lovely light~Edna St.Vincent Milay