I have a high self-esteem and all but I think I’ve taken it too far to the point where I’m just being plain stuck-up and vain.

I’m not completely sure, but high self esteem is just being confident in yourself in whatever you do.

Being stuck up or vain would be in how you express your "high self-esteem" to others. You could be in the "I’m better than the rest of you" mood = vain. Or in the "We’re pretty evenly matched, (even if you’re not) let’s give it our best shot" mood = high self esteem without being arrogant.
You can have self-esteem, but not brag about it, or think you’re better than others, or just be in a way that no one would like. Just keep things to yourself and keep your feet on the ground and you won’t be stuck up.

I hope this helps.