Assalamualakum, I was born as a Muslimah and still am one. Recently I have wanted to get closer to Allah and increase my imaan. Over the years I have not been a good Muslim, in my past I used to skip my prayers (I’m embarrassed of that) and I lied a lot. Though I have repented I still want to be a better Muslim. Should I say my shahadah again?
If you mean shahdah as in to re-become a Muslim, I would say you never left. Instead, what I believe you're asking is how to get on the good side of things now that you're grown and more mature.
In this case you can't rewrite the past. You can take the right actions moving forward. Whatever happened in the past, make du'a for taubah (repentance) asking Allah for his forgiveness and mercy, that's the best you can do. There are many du'as one can learn asking for Allah's forgiveness.
Reciting the shahdah is dhikr and a testimony of your faith, you can do this as many times a day. The more the better.