To be honest, the age of this horse does make me think that it might be a good idea to sell. But I also wanted to ask... has this horse forcefully bucked or reared, or caused you to fall off? Has his behavior been unsafe? If yes, I would absolutely sell. If he hasn't seriously tried to hurt you, I would have a long talk with your trainer because the question is then: do I have the time to put into this horse, or do I want something a bit more finished?
Here's what I think the problem is: this horse has your number. There isn't trust or respect, which makes it easy for him to spook and misbehave with little regard for what you think or tell him. It also makes it hard for you to have confidence! And that becomes a vicious cycle.
It takes a lot of work to earn the trust of a horse so that they look to you when they spook, instead of spinning and bolting away. When I bought my gelding (also in Decmeber!), he seemed rather "bombproof." Needless to say... no horse is. ;) But after putting in time and effort on the ground and in the saddle, he will spook in place at most and wait for me to let him know if it's okay to move forward. Sometimes he'll refuse, and I'll whap him on the butt and he'll give a threatening crowhop-- but as long as I don't lose my confidence, he'll back down and we will move on happy and safe.
With the help of your trainer, you might need to spend some time on groundwork and share a few "arguments" in the saddle to prove to your horse that you are a good leader (even if you have to fake the confidence at first)! The question is whether you want to go down this path or start new with another, more seasoned horse.