Should i lengthen it?
Not unless it gets better. Meaninglessly long is bad
I still think 390 is a little too short, as generally most essays that short (yours might not fall into this category) are not very good as they are vague and do not explain the significance or how the experience relates to current goals. Instead of adding new stuff, try expanding upon your current ideas.
I would try a way to add length in a meaningful way.