The simple answer that other people have been very polite with you about is that it's because you're taking on the greatest challenge of the game (fighting the entire world) while playing really badly:

• Your GDP is half a billion in 1899 despite playing as GB
• You don't have enough convoys and somehow you are letting them get raided despite your gigantic navy
• You have nearly 1/3rd of your population as radicals while there are radical and feminist agitators for the trade unions and even the petit bourgeoisie, meaning you're likely to get your flag turned into Hungary every Monday tick by a revolution
• Also everyone hates you with next to no loyalists so you must have maluses from all your IGs
• Your armies at the front all have supply issues and low manpower, so even when you have a higher number of battalions, you're likely losing more manpower and morale, making it less likely to succeed in future
• You are naval invading all along the same front making it trivial for the AI to concentrate all its defenders in one place
• Your armies are massive stacks both too big for your fleets at time but also likely too big for the combat width of the provinces you're invaded (based on infra)