Manual transmission hard to shift when cold

If it's specifically 1st and 2nd, also check your clutch pedal free play and the shift linkage bushings. Worn shifter bushings are super common on 2008-2014 WRXs and make shifts notchy. A Perrin or Kartboy short throw shifter bushing set is  and takes 15 minutes to install. Night and day improvement in shift feel.

Very common on WRXs and most manual transmissions. The gear oil is thick when cold, making synchronizer engagement harder. Two fixes: 1) Switch to a synthetic 75W-90 gear oil if you're not already running one. Motul Gear 300 or Redline MT-90 are the enthusiast favorites for WRX transmissions — they flow much better cold than conventional gear oil. 2) Rev-match your downshifts and double-clutch into 1st when cold. The syncros on WRX 1st/2nd are undersized from the factory.