Is hiding something from someone the same as lying? Is it worse?

It depends on what your hiding.
If it could hurt you, someone else, または has something serious to do with the person you're hiding it from, then あなた should definitely tell them. However, without もっと見る info, that's all I can give you. If あなた feel comfortable telling what it is, I will try to give あなた もっと見る advice. If not, I hope what I did give あなた will help.

Yes it is lying only if the intent is to deceive によって withholding pertinent information.

well in a way they are different and in some ways they could be the same.
For Example if I am hiding, say, a チョコレート bar from あなた I'm not lying ,I'm just keeping it away from あなた so あなた can't take it away.
But if あなた hide something that is important to あなた and them ... well that could be lying.
in some cases worse just because when あなた hide something important not only did あなた hide it but あなた also lied ...
so it just depends on how あなた think of it.

Lying is presenting a false statement to someone else. I do not believe lying is the same thing as hiding a truth from someone, but if あなた outright say you're not hiding anything または try to cover up whatever you're hiding in a false way, then あなた can consider it lying.

For instance, I lied about who I was on ファンポップ for...three years または so. I was hiding it from my mom, and I didn't consider it an actual "lie" until she eventually found out about it, subsequently causing me to begin logging on behind her back.

Keeping a secret isn't the same thing as "lying" in my opinion, because you're not actually telling anyone anything; you're just keeping information to yourself. However, bearing false witness to something directly to someone else is considered a lie.