I used to work as a certified optician. If you had trouble adjusting, you should try a better quality lens and/or have them check your measurements. The cost is high, but your quality of vision will be sooooo much better than with a lined bifocal. People think that they will get more lens area to see through with a lined bifocal, but they are incorrect because the Rx is not correct over the entire surface area of the lens. When you start with your lens blank, which is a giant circle that will be cut to fit the frame, the Rx is in the center of the lens. If you deviate from the center, you create prism in the lens, which distorts the image. This means that if your measurements are off or if you move your glasses from where they were measured to sit you will experience distortion. As a result, when you have a large lens in your glasses you do not actually have that entire surface area to see through. Your brain/eyes automatically figure out where to look through, though improper measurements may make this impossible. In addition, the bifocal area of a lined bifocal is accurate at the top center area of the little half circle at the bottom of the lens. Based on your measurements, the half circle (think of a bisected lens blank) is positioned below your eye (a little closer to your nose because your eyes converge when you read) so that you can look down and read. It does not matter how large the bifocal area is because you only have a small area that is usable for your Rx. The only exception to this bigger-is-not-better rule is a pair of glasses that is too small. Usually, this does not happen unless you find a really small frame.
My advice is to upgrade to a better lens and to make sure that the measurements are correct. I know that it is hard to spend the money, but think of how often/long you wear your glasses. It always amused me that people would try to cut corners on their glasses when they would go blow a bunch of money at the mall/Walmart. Also, try to find an optical store that will remake the glasses until you are happy. At the location where I worked, our manager would remake the glasses as many times as was needed when people had problems. This is obviously not true everywhere, as the first location where I worked was different (and they are both part of the same organization). The reason that no-line lens are so much better is that they provide you more natural vision than the line, including a computer distance area in the middle (kind of like a trifocal).
Hope this helps.