And maybe one day he’ll be all the things you need him to be. He’ll be kind and caring and considerate. He’ll open your car door for you and carry your books to class. He’ll give you roses for no reason and take you out for last minute dates. He’ll bring you breakfast in bed and kiss you in the rain. He’ll dance with you in the shower and get drunk with you under the stars. Maybe one day he’ll be all the things you need him to be. But not today. Today he is young and foolish and reckless. Today he is selfish and not ready for love. Today he wants to drink and dance and live like he’s immortal. Today he can’t be all the things you need him to be, all the things you hoped he would be.
Jim’s biggest fear is losing the few people he cares about, and throughout the whole season you see that fear grow and grow, to the point where he has nightmares about it. Claire is nearly hit by a truck right in front of him. His mother nearly dies in his arms. Aaarrrgh! does die. Jim’s pixie-induced waking worst nightmare is his mom, Toby, and Claire turned to stone, and it cuts him so deep for a moment he believes it’s real and his legs give out in front of his mom and you see the agony in his eyes. He lies to her again about his Trollhunting because he’s scared she’ll get hurt again. He lies to his friends about taking on Angor Rot with Strickler because he’s scared they’ll get hurt. Gunmar threatens to slaughter everyone Jim loves because Jim killed Bular. Angor Rot tells Jim the reason his loved ones have gotten hurt at all is because of him, and Jim believes him because Jim has blamed himself the entire time.
Jim doesn’t decide to go into the Dark Lands alone because he has some newfound overconfidence in his own abilities or thinks he’s invincible or something. He knows he’s screwed without his friends. He’s the one that’s been advocating for their support this whole time when the past Trollhunters have argued against it. He knows he’s not ready to face Gunmar. He gets his ass whooped when the past Trollhunters try to teach him how to fight off multiple enemies at once because Gunmar’s army isn’t going to attack him one at a time. He knows. But he sees Toby and Claire standing beside Aaarrrgh’s stone corpse and thinks about Gunmar’s threat and how easily Toby and Claire could be next, and its too much. He’s scared and he’s sorry and he cries. “I can’t lose any more of you.”
This is not someone who believes they can win on their own. This is a kid that’s fucking terrified of losing people and desperate to prevent it from happening again. This is a kid who’s guilt and anger and most of all terror have convinced him this is his only option. He has to go alone otherwise more people he loves are going to get hurt or killed because of him and he can’t live with that.