My last big closet cull involved putting my clothes into three different piles. Pile one was "love wearing this", pile two was " like this" and pile three was "don't like this anymore". I tried not to think too much as I sorted the clothes; went with my first gut reaction.
After I sorted them, the "love this" pile went right back in the closet. The "don't like " pile was put in a bag to donate. Pile #2, the "like" pile, is the hardest to deal with. Those are the clothes you need to ask yourself "what is it about this that makes me like rather than love it?". Is it the fit? Is it worth it to have it altered? Is it itchy? Does it fit with the style I'm going for? Etc. That's the pile I divide up into "alterations", "keep in box for a while", "return to closet" or "give away" piles.
If I'm not sure about letting something go, I'll put it in a large shopping bag/box and save it for however long I need to. Usually after sometime, I'm ready to let it go but occasionally I'll pull it back out and be glad it's still there.
Good luck with your culling