I haven't had a thyroidectomy, but from what I know from those who have, it does take a while for the incision to heal.
You should be about ready to go back in for a post-surgery check up, but if you feel that there's anything abnormal about the incision, you should contact your doctor and ask to go in sooner.
Have you been put on replacement medication yet? If not, that could have something to do with the feeling you're having as well, since you could be having some hypothyroidism, without a thyroid to produce the hormones you need.
Have you had any lab tests (TSH, Free T4, Free T3) done since your thyroidectomy? If so, it might be helpful if you could post those results and their corresponding reference ranges so we can see what your levels are.
I had two thyroid surgeries last year, one in June 2018, one 2 months later in August 2018 (through the same incision site). I'm pretty sure my first scar from June was pretty stiff for at least a month, like a hard seam. Eventually, by the time of my second surgery, it had softened way up and was feeling a lot better.
I also had a lot of inflammation after my first surgery - I had a lobectomy and then the rest of my thyroid removed later, so I'm not sure if the inflammation was from the surgery or from my Hashimoto's inflammation (and lots of swollen lymph nodes), but it was definitely swollen around the scar in June for probably a month or so.
hope you feel better - if you are worried about the hardness, it should go away, mine did. I feel like after my second surgery it took even longer, but now a year later, the scar is barely noticeable unless someone is looking for it, and it is very soft and flexible. I still put sunscreen on it everyday - I have fair skin so I'll probably just do that just like sunscreen on my face everyday from now on.