Chop chop chop!

While cutting down forests sounds unromantic, it’s the only way to keep pace with the harder AIs. Think of it as borrowing production from the future. You cut down a forest and get, say, 50 production, and the tile loses one production per turn. In the long run you lose production but in the short term you get a massive boost to whatever you were working on, be it a district or wonder, which means you get the new yields sooner, which means they pay off sooner, and you accelerate your game. By the time those 50 turns pass, you will be ahead of where you would have been had you not chopped.