Pretty sure a lot of people in the comments have already suggested helpful online resources however I'd recommend you start right where you left from; your mid/

Pretty sure a lot of people in the comments have already suggested helpful online resources however I'd recommend you start right where you left from; your mid/high school textbooks. I'm kind of old-school and I think a pencil and paper approach w/o online distractions is how you really get closer to understanding math. Of course you can use various tools (e.g. Wolfram Mathematica) for any fancy visualizations and maybe some tedious calculations. Just don't rely too much on them while learning the fundamentals (don't skip these boring "find the following indefinite integral" problems).