Tips on switching from a Flute to a French Horn quickly ?

Of all the br, F-horn is unquestionably the most difficult and frustrating to learn. It is not you- it is the horn. Most good F-horn players were good trumpet players first. Don't give up playing altogether but try to get back to that which you already had success at. Flute transfers well to baritone saxophone and most kids are afraid of that huge beast. But it is a really fun one to play. Aim for that.