I'm not a game dev but I heard from actual game devs is a little trick that I think will make this game a little less rage inducing.
In platformers, there are usually 1-2 pixels of empty space of which you can stand on next to the actual platform which makes jumping a lot easier and fixes the 'slipping off the platform' problem.
Also I think that one mouse or fall makes you start from the beginning is a little unforgiving.
Very well done for your first game though!