How can I improve my long term gains?

Hello everyone, I am 21 years old and plan to invest between 200 and 400€ per month in some ETF's for the long term.

I use Flatex if that matters.

ISHARES GLOBAL CLEAN ENERGY ETF (100€)

ISHARES NASDAQ 100 ETF (200€)

LYXOR S+P 500 ETF (100€)

This is my current setup.

I am currently 1850€ in and am already at a 10.5% plus.

I plan to keep investing for atleast the next 20-30 years, so I really
do not want to make any mistakes now that i may regret later.

Do you have any ideas or information on how to
invest smarter? My plan is to retire earlier than
65, which is standard in my country.

Thank you all for any tips or tricks xyou may have.

Please excuse any grammar mistakes.