You have to throw games a few times when the game starts getting too hard. Unfortunately the difficulty level of the game is automated, but it changes based on your success and failure rate. The more you succeed the harder it will adjust the enemy AI, the more you fail missions the easier it will adjust the enemy AI.
Other factors can modify this further, like the MMR of the other 3 players you are in a group with and the level disparity between you and the antagonist, should you get an antagonist. But generally speaking it will adjust.
Don't be afraid to throw games when it gets too hard, even top high level highly skilled players have admitted on these forums that even THEY get to the point where the enemy AI is ridiculous and extremely unfun and IMPOSSIBLE and they are forced to throw 2 or 3 games (I would say at least), to get back to a more acceptable challenging difficulty, one that is more possible to complete.
Unfortunately this is a grinding mechanism to gate progress because if we are too successful in our games then we may be gaining rewards "too fast". Now this most likely has nothing to do with the Developers, like the game director, animators, etc... and probably more to to with the publisher, marketing department. This is very common in gaming business where a bunch of executive suits who have no idea what good gaming is makes horrible greedy discussions for a game and the developers can't do anything about it. Prime example was Evolve. Turtlerock was an amazingly creative development team and made an extremely beautiful and original game, but 2K games completely ruined it with greed.
There has been a lot of cultural pushback more and more from the gaming culture in general towards this kind of dishonest "gating" mechanism, especially in conjunction with micro-transactions (certain coountries, like Belgium, and more are even criminalizing it).
So we can just speak our mind here and let them know. Hopefully the developers will let the "suits" know in their ivory towers and it will pressure a change. Again, I don't blame the developers at all, they are what make this game great.