tl;dr: read the FAQ :)
Just always remember: What will you do when Bootstrap does not provide what you need? What if Bootstrap becomes outdated (like what happened to jQuery)? How will you go on from there? What if people start hating the Bootstrap style and something new and awesome hits the market? Will you go and painstakingly learn the new library? What about small projects? Bootstrap has a lot of features, but what if you only need very very few of them? Will you set up a build system which removes all the clutter? Or will you send everything to your clients, so they have slow load times and you use up their mobile data volume?
Here's my advice: NEVER START BY LEARNING A FRAMEWORK.
You should definitely take a step back and try to leave Bootstrap completely out of your next few projects. Most stuff can easily be done without Bootstrap. I'd even say that if you get to know CSS and good CSS architecture, it will actually be easier to just do stuff without the framework - instead of using it and then stripping stuff out or modifying and extending it to suit you needs. I highly recommend taking a look at normal CSS and learning BEMIT and ITCSS (combine them!), which are not frameworks, but architectures (think OO or FP for programming languages). That way, you will be able to write efficient, maintainable and scalable style code without 3rd party tools.
Jason Knight
The less code you use, the less there is to break