I can only guess you used construction grade lumber here. if that's the case the title of the thread seems a bit misleading as for my entire life there has never been a time when construction grade lumber was straight and true.
I don't know how to put this nicely, but I'll try my best. It's not the lumber's fault if the carpenter doesn't know what they are doing. There are a number of ways to true-up lumber, or ways to avoid having to do so to get a level surface. for a beginner I would recommend using strips of plywood rather than using 2x lumber as it will typically be a lot easier to work with and won't bow or twist. You can cut nice, square, strips with nothing more than a circular saw and a long piece of scrap steel or aluminum and a couple clamps.
For the tables you have already built, I'd try using shims under the surface to level them somewhat. Once the surface has been leveled you can use a sub-floor adhesive, construction adhesive, or expansion foam to fill the gaps between frame and table top and hold everything in place.
JGL
