Beginners always think this. You have not learned to use your feet. The 50mm is the perfect lens for most portraits on a crop-frame body.
One good exercise is to place just the prime lens on the camera then shoot with it all day or for several days. You might mess many shots you could have gotten but in the end you ail have just as many good shots no matter which lens you choose, you will just have
different good shots. The purpose of the exersie is to learn to see like that lens. to learn what that lens is best at., to see a subject and know quickly where you will need to stand to get the subject, light and background. For many years the 50mm was the "kit lens" and that was all people used. They made fine photos back then, makebe even better ones than we make today.
With a zoom lens you don't learn to move the camera position, you get lazy. The correct way to use the zoom is to (1) move the camera relative to the subject to get the perspective you want, then (2) adjust the zoom for the angle of coverage you want. Almost every beginner skips step #1.
I'd say if you have not yet shot at least 1,000 or so frames with a lens do NOT buy another one. Wait.
Here is how to decide on a new lens: Look at your photo library. If you own three lenses then you should have thousands of images in there before you are looking for a 4th lens. So sort the photos by f-stop. Are many of them shop "wide open" if so then you could use a faster lens. But if so your shot are at f/8 or f/11 you don't need a faster lens. Now look at the ones you shot with the kit lens. All many of then at one end of the zoom range, either 18 or 55. If so then you can use either a longer of wider lens. But if you are mostly using 35mm then no. Next look at the light level, if shooting in low light in many shoots then an IS lens might help.
The other thing to do is look at what you think of as your best work. Look critically. How could these be better? Is there even an equipment problem. At this point I'd guess if you have an equipment problem it is with lighting and you could use light defusers, reflector cars, bounce flash or other better lighting technique in you "pepple shots" and likely in shots of small objects.
I did this analisys once and ended up with a stroboframe flip flash bracket
http://www.amazon.com/Stroboframe-Camera-Flip-Flash-Bracket/dp/B00009UTLU
I'm NOT saying to buy one of these but to LOOK AT YOUR WORK and based the next bit of gear on some IDENTIFIABLE problem
Find some photos you like on-line or in a book or magazine, shoot 100 frames in effort to copy that style, go home and select the best five. See how well you did compared to the professional images This gives you a goal and a standard to compare with. Copying the style of the master is how artists have learn for centuries. At some point you see your work could be better and you fix it. At this point almost certainly the "fix" will not require a new lens.
Shot about 100 frames keeping five at least once a week. Those 100 frames need to be all with the same goal, some "look" you are trying to capture. Run you full post processing work flow (whatever that is) on your 100 images, likely that is fist to select the 20 best and work with those then dowselect the best five and do more work with those and finally fine tune those five. Then look again at the model photo from the book and plan the next attempt.
That 50mm is really best for "street photography" because you don't waste so much time fiddling with the zoom ring. You see the subject and then have learned how to pick the spot you need to walk to. After a while you KNOW that spot without looking through the camera. The camera handles VEY fast with a fast prime, the AF works quick and you can most always work with a fast shutter speed and existing light. So you can have the shot just as fast you you get yourself to that spot. The key is learning to recognize where you need to be. And when things are moving it is learning where you will need to be in one minute.