Choosing a Madhhab

Assalamu alaikum. I need guidance about missed prayers and choosing a madhhab.
I grew up learning Islam from a teacher who previously told me that she follows the Shafi‘i madhhab. However, I have realised that I have followed a mixture of Hanafi and Shafi‘i practices.
I reached puberty at around 14. From approximately 14–17, I prayed some of the five daily prayers but missed others, and I do not remember exactly which prayers or how many. I did not understand the rulings properly at the time. My teacher advised me to repent and make istighfar for those missed prayers, rather than make them up.
I am also unsure whether I need to make up a prayer if menstruation begins after its prayer time has entered. I was told that I do not, but I want to understand the ruling correctly.
Could you please advise me on how to approach these past prayers, whether I need to make them up, and how I should choose or follow a madhhab going forward? I would appreciate guidance that takes my uncertainty and lack of knowledge at the time into account.
Jazakum Allahu khayran.