Most Helpful Girl
If you never do the things you want because you're scared that someone will laugh and say mean things, you're not going to get a lot done :/
Chances are, no offence, that a lot of people thought what you did on youtube was dumb. There are always going to be those people. But hey if that many people liked what you were doing well you gotta be doing something right!
I don't know how you can stop caring about what he thinks, everyone deals with that in their own ways, maybe try googling "how to not care what people think" and see what that says. But it goes without saying that you shouldn't let a shitty ex hold you back! You'll regret not doing it in a year or so when you really don't care what he thinks anymore.
TBH thinking about it there probably isn't any magic way of suddenly not caring, you literally just need to decide that you're going to do it, your feelings won't have changed or anything, you just need to decide. And you you should carry on with your youtube!