MS can definately be around, yet not show up on an MRI. Did you have the MRI done of both your brain and spine? Because lesions can show up in either place. If you only had a brain MRI, it might be worth it to have one done of your spine as well. Sometimes MS only shows up in a spinal, so might be worth it, at least to ease your mind. Either way, you deserve some answers. Don't give up, you know if you aren't feeling right, so keep seeing doctors until someone listens to you. I can't stress this enough, because if it is MS or something else serious...you need to move on to the next step. In the case of MS, you'd want to start treatment ASAP to slow done the progression of the disease itself, and to slow time between flare ups.
I've read here and on other MS forums of many people who had symptoms for years, and the doctors would ignore it and tell them the MRI showed nothing, then later to find out they did indeed have MS. Also, keep in mind it isn't necessarely MS what you have, but something else. There are other health issues that have similar symptoms as MS, so your doctor should be testing you for those as well.
Take care, and I hope you get some answers soon. God bless!