What would one do if they make a mistake during salah. For example I think I forgot to read a surah so I stopped praying and started my salah again. I know you're not meant to stop but I though my prayer would be invalid, have I committed a sin by doing that?
This is a highly complicated topic. In general one performs what’s known as sajdat as-sahwu when one makes a mistake in the prayer, where one performs two extra prostrations right before the end of the prayer (before saying assalamu alaikum… at the end). In the Hanafi school, you say the first salaam, then perform the two prostrations, then say the left salaam.
The necessity of this act of doing extra prostrations is itself debated, the Shafi`ite and the Malikite school say it is voluntary, not obligatory, meaning that if you make a mistake, you can continue your prayer as normal and finish it without issue.
Also note that all of the schools agree that reading a surah after reciting fatihah in the first and second rakaat is not obligatory, but voluntary.
Since you stopped the prayer without knowing any better, I wouldn’t say it is a sin, but it is better to ask God’s forgiveness nonetheless. If you need more specific information on this matter, I recommend that you ask your local imam.
I sat down in the third rakat and recited the atayat soorah is my namaz invalid?