Sorry if you've said, but have you done other groundwork besides longe him? Groundwork comes first, imo. You learn to be the leader, to move the horse's different body parts, & to partner up with horse, which transfers to saddle time.
I must add that I think your trainer's been too "stiff upper lip, fake it till you make it" with you & your fear: fake it till you make it doesn't work with horses, since they're masters at reading emotion & intent. Guess you know that, because after mounting up all those times that you didn't want to, you're now wanting to sell him. Better if you'd developed a conversation with him via groundwork, which'd've upped your confidence for riding.
Before you sell him, I'd get some groundwork instruction, like Parelli 7 Games, & learn those with him. Best of luck, & I think that poor OB has done really well, being so green & having to deal with a green owner, & ridiculous visions like sheep costumes!