I'd try and find exactly where the pain is coming from first. If 20 lbs db's are hurting you...something ain't right, so you might need to rehad something. Most likely it's an AC joint thing or rotator cuff. But you never know there's always tendonitis, bursitis, bone spurs, tears, ligament stuff, etc. So again, find out what's causing the pain. If it's ac or rotator I'll tell you what I did and am still doing. I'm just getting over some ac and rotator stuff in one of mine. I'm going to a chiropractor who's experienced in working with bodybuilders and powerlifters and he's helped stabilize my ac and my scapula to help with the rotator.
But the most important thing for me was...rest. I didn't do any pressing movements for chest or shoulders for a couple of weeks and began doing rotator cuff exercies. Then I, started working chest presses back in and found which angles (incline,flat,or decline)I could handle better. But I lifted very light and slowly worked back up.
When I started direct shoulder stuff (about a week or so after starting chest presses) again it was light db's and cables, with NO OVERHEAD PRESSES. A few times I did upright rows during this time...bad mistake. It flared the ac up for a couple of weeks. If you insist on not laying off shoulder presses, stick to the alternate db presses were you start with your arms in the same spot you'd end a close-grip pulldown; it's a little less bothersome to the shoulder. But, honestly I'd stick with side laterals and front raise till you get it under control. I'm sure there's many on this site who can attest to the problems an unattended pain in the shoulder can lead to.
After about 4 months I'm 85%. One of the best things I did a couple of weeks ago was take another week off, it did my recovery wonders. My bench is back with only minor ac pain for a couple of days after and no rotator problems. I still go easy on the overheads, doing only one a workout and sticking to the one I mentioned earlier or Arnolds. Just be smart, rest it, don't be scared to lift some wussy weight if you got to, and be patient. Hope that helps at least some. Good luck.