I think the metaphor you’ve chosen is unhelpful. So, you build a wall in your mind, and have to actively send your thoughts back out of sight while they’re trying to invade. You’re under siege in this metaphor, and only barely holding out. So when you stop this meditation, of course you’ll be overrun! You’ve told yourself that that’s what will happen simply by choosing this as a metaphor.

But other metaphors are possible, and might be more useful for you. You could imagine your thoughts as small animals, ducklings maybe, waddling around and falling over, and they’re sort of cute but you don’t have to pay attention. They’ll just waddle away again on their own. Or, more traditionally, consider your thoughts to be drifting clouds through the bright empty sky of your mind. Some drift by slowly, some quickly, but they always just drift by on the wind, you don’t have to do anything. You don’t have to name them or see what shape they are, they’re not important. You just keep looking at that beautiful blue sky, which represents your mind in this metaphor. You mind is always still and vast and pure and blue. Thoughts are just weather, ain’t no big deal.

I think if you try out this kind of method, which considers thoughts to be simply uninteresting and unimportant rather than a hostile hoard embattling you, you may have a more lasting peaceful effect.

I also second Rock Banana’s suggestion to look into mindfulness meditation, which may have a no-mind kind of effect but that’s not necessarily the ‘aim’ or ‘purpose’ of the meditation. (And speaking from personal experience, that effect is likely to happen after a day or so of continual practice at a retreat rather than in a short session. YMMV though!) Some styles of Zen sitting have a ‘no purpose, just sit’ kind of attitude that may also be helpful getting out of this “Besieged!” feeling.

Good luck, man!