I'm not surprised that it's taken so long given that you are from Nigeria. It's not by accident that I mentioned Nigeria above (before knowing which country you are from). Nigerians have a bad rep in Thailand (love scam, call center scam, and other online scams). It's not your fault. You will have the occasional old foreigner shimming in with a personal anecdote and anything under the sun to ignore systemic discrimination. Ignore them, they live in their ivory tower.
It would be easier to come to Thailand on a tourist visa and start the process over. Need a lawyer to make sure you don't get screwed over while here. Thailand have had his fair share of love scam and internet scam from Nigeria and other African countries therefore they often are very paranoid about allowing people from Africa to come here. Being here and involved in the process directly with lawyer, and access to the immigration office, etc, would help a lot.
You are black, and African. These are the reason it's taken so long. If you were white from the UK, you would be okay already. Yes, they are taking longer because of that and the chance of rejection is higher. No, I do not think that's it's fair, but it's the world we live in.