Best things to cut bitterness I've found with collards -
1. Buy organic, not conventional. EVERY batch of non-organic collards I get taste way too bitter for me, organic far less, so that's the only way I'll buy collards now, even though it costs twice as much.
2. I try to cut out the veiniest parts of the collards rather than cook them all up. This means it takes me about 20 minutes to cut and prep abou 4-5 bunches, but they tend to be less stringy and a bit less bitter if I cut around the big white veins in the leaves and focus on the good green stuff. A bit less fiber, perhaps, but when you're eating as much of them as I do, it's not a worry
3. I don't always cook them right away, I often let them sit for 2-4 days in the crisper before I cook them. Seems that if they have a few days to sit first, they end up less bitter, and they don't usually wilt quickly so a few days doesn't seem to do any harm.
4. A little vinegar goes a long way with cooking collards. We usually have rice wine vinegar at home, a few splashes here and there in a big pan with about 4-5 bunches stewing up and it helps take the bite out.
5. It's all in the seasoning. I usually make mine either with cajun seasoning spices, a dash of liquid smoke and lots of coarse black pepper, or, with Indian seasonings, as things like tandoori seasoning or tikka really suit collards well. Just a cup or two of water in the pan, add seasonings in as the greens wilt a bit under medium heat after about 5-8 minutes, and they come out great every time. Of course, if they're still a bit bitter, a few dashes of your favorite hot sauce mixed in will fix that quickly enough