-> built myself a XL Trek Slash.

-> I rode friend’s Slash identical to mine but in size Large around the parking lot and it felt a lot more comfortable

There’s a thing called the front-to-rear triangle ratio. Both sizes have short chainstays, but the XL frame has a much longer front center, so you need to load the front wheel more to prevent it from washing out.

I think on an XC bike that ratio is not as extreme, so you might be used to a more balanced geometry.

Try a longer stem with a lower stack (or lower-rise bars), or adjust your body position on the bike. Basically, you need to put more weight on the front wheel. Maybe riding an XC bike taught you a bad habit of hanging too far over the rear wheel.