Eren picked a lot up from training with Annie and fighting her as a titan. Before Berthold showed up, it was that knowledge that essentially defeated Reiner. He just needs a chance to gain more knowledge and learn more skills (from more people).
Also, the titan body is very light for some reason, and heals itself. Once he's at full strength again, he will have room for error-- as long as he doesn't allow himself to be captured again. He also might be building stamina naturally. The more he uses titan power the longer he can hopefully last, and the faster he can recover. Annie had miles more stamina than Eren did, remember. He was way tired out by their little bouts, while Annie was always able to keep right on going (even if she was tired in the morning after the first time, that's impressive-- a testament to her fighting style I guess). Reiner and Berthold have special skills that wear them out after battle, clearly. The only thing they have going for them as of the latest chapters is that Reiner totally mangled Eren and Ymir so they have to regenerate their bodies or they'll form titan bodies too weak to survive. All four of them were in fights as titans before the current situation so otherwise they should be comparable aside from the "special skill" factor.
Someone in the Annie vs. Mikasa thread had a good point about the impression being given that it's kind of implied Annie has
memorized enough battle situations to be able to respond instinctively in the heat of the moment, while Mikasa has the
creativity to deal with just about any situation. I bet you anything Eren is in between the two of them on this personality trait. Somewhere between Annie's efficient "rote-learned" fighting and Mikasa's cool-headed adaptability. He has a stronger will than either of them (both his greatest strength and weakness, lol), and he can apply it to memorizing moves like Annie, but also building on that theoretical knowledge in order to get as "zen" as Mikasa with her fluidity and creativity.