I'll bet your horse knows you aren't interested or not enjoying his company. That would explain the escalating issues when you're on his back. Same thing could happen with another horse if you don't learn how to train the bad behavior out of him, but it might not.
There are about 25 horses at the barn where I ride. Out of those, there are really only 2 that are sweethearts all the time, and one of them is mine. The rest can be buttheads when they feel like it and they regularly test riders and need reminding who is the boss. It's in their nature to always be checking to see if you're up to lead-mare status.
What are the chances you're going to find one of those perfect horses if you sell the one you have? Probably not that good. People tend to hold onto those horses, not sell them.
If it were me, I'd give it one last push with your current horse with a trainer there helping, and work on the horse's attitude under saddle. And don't expect too much at once. My horse was nervous the first few trail rides we did, but she eventually got better and better and now she's great.
You can't expect them to just get over their fears without a lot of desensitization first. And sometimes that desensitization takes a while. I'm not saying you aren't aware of this stuff. But sometimes even with Clinton's DVDs, you can forget that not all horses catch on as fast as his do in the videos. :)
No. Just no. "Horse knows you aren't interested or not enjoying his company?" Have no idea where you came up with that, but trust me, horses do NOT care if we are enjoying their company, nor do they care if we are interested. They are horses, not a BFF.
Every single horse will try this on someone that doesn't have much knowledge about horses. And that same horse will act like they are supposed to the instant an experienced rider mounts.
This horse is not working out and has the beginnings of some serious issues that OP is not ready for, in any way, shape or form. Getting light on the front end is not a good thing.
Desensitization is smoke and mirrors, nothing but busy work much of the time for a human. And isn't needed nearly as much as people do it. And rarely is it fear as much as it is a horse that knows a rider doesn't know what they are doing.
And horses are out there that are kind, tolerant and easy going. This one isn't.
Also need to check handling skills, as could be adding to this.
But any horse, even the most gentle, bombproof horse will act up if it feels the human can't make it behave.