What is your TSH level? How often do you get it checked?
It took me a long time to come even close to a dose that worked for me. I kept asking myself and the doctors, "will I ever feel normal again." Some people find that they can just take their pill and go on with life. Other people (like us), are much more sensitive to small dose differences or tiny fluctuations in our medication.
I am also taking both Synthroid and Cytomel. Personally, I found that I can think much more clearly now that I'm on both. I couldn't concentrate before. I am a pretty articulate person, but I had trouble coming up with words to finish a sentence.
If your doctor is following the guidelines for cancer suppression, he or she wants your TSH to be less than 3 or maybe less than 2, depending on your risk for cancer recurrence. The lower your TSH, the lower your risk of cancer regrowth. Some people feel great between 1 and 2. For others this is too hyperthyroid, which can bring on symptoms, including fatigue, aches and pains, difficulty sleeping, anxiety, etc. In addition, prolonged hyperthyroidism can cause osteoporosis. Unfortunately, we as patients need to make the ultimate decision about what risks in one direction or the other we are willing to accept.
Also unfortunately, it looks as though you are having other on-going concerns with possible regrowth. I'm assuming you went through Radioactive Iodine therapy, right? Have they done any thyroglobulin tests in addition to the ultrasounds? This is a hormone secreted by any thyroid cells. If there are any present, they should secrete this hormone. My doctor says I need to have a thyroglobulin test every year for the rest of my life. So far, I've been clean. I haven't had an ultrasound since before my surgery, but there is probably a specific reason why you are having them done.
Have they done a biopsy on your nodules? If so, what did they say? Is the doctor taking a "wait and see" approach and going to send you for another ultrasound in six months or so? Or does he recommend surgery?
I don't know how worried you should be. The majority of thyroid cancers are very slow growing and treatable, even if they recur. What type did you have?
Sorry about the long post. I start talking about this stuff and I just can't stop.

Please write back with more information concerning my questions and maybe we can be of more help.