Since your doctor has already recommended an OTC fiber supplement, I'll give you my experience.
Look at the calories on the back on the product. Some are loaded with sugar. Look for a sugar free product.
Some products may cause more gas than others. I, for one, cannot tolerate Psyllium Seed Husks as found in Metamucil, but it does not bother my husband. Try taking it on the weekend!
If you cannot manage one glass of liquid all at once, there are capsules on the market now, but I suppose you have to follow those with a glass of liquid anyway.
I find Dextrin, as found in Benefiber, more tolerable for my sensitive GI tract. It dissolves completely and I stir it into water, unsweetened iced tea or sugar free Kool Aid. I take this 2X daily with no ill effect.
Even if you take a couple of doses of OTC fiber supplements daily, you're only going to get about 6 grams of fiber. You will still need more. Be sure to post all your food on the nutrition tracker and check your fiber intake. Fiber should be part of every meal to get you to the 25-35 grams you need every day.
Folks are right: Go for whole grain cereals, breads, pastas. Eat whole fresh fruits/vegetables with every meal and as snacks. Apples, for example, with skin have as much fiber as a serving of Metamucil, but bananas have virtually none. Check the nutritional information of each serving of produce as you consume it so you'll know what's helping you to get the fiber you need for better health.
Good luck. Keep us posted!