I can add/subtract 3 digits in mental math but i cant do any any multiplication/division past 2 digits idk why it seems I forget or something, i wasnt the best at math but i want to atleast be good when I pass, I am 17 in 11th grade I can do long form of multiplication/division addition and subtraction in paper effortlessly but why can't I do the same for mental math? I want to be good at mental math just like how I can do in regular paper

Hey guys, so about 2-3 months ago I decided that I will try to master calculus 1 while self studying and having never taken calculus in high school. I’m still attending highschool tho. Since then I’ve made massive progress I would say. I’m currently reviewing everything I’ve learned in calculus 1 and polishing every skill. Looking back I think this is the best decision I’ve made in a couple of years, honestly. I have also decided that after I master calc 1, I will continue on to calc 2 and 3. And after that I’ll just keep learning math until I complete a full mathematics course that a math minor/major would. I have no idea how long this is going to take me but I’m sure I’ll love every second of it. I was wondering, is it as easy to find free lectures online for further maths such as real analysis as it for calc 1-3? I would greatly appreciate if you guys were able to drop a few link might help my endeavors. 👋