kenmore drain pump assembly

Posted earlier about no draining. Figured out it was, of course, a plugged drain hose. Unfortunately I got underneath and unscrewed a bunch of stuff before I figured that out, so now the drain pump leaks. I took it apart and can’t get the O-ring to seal in between the two halves. Does anyone have ny tips, or have I ruined it by taking it apart? Should I just order a new drain pump? If so, I cannot figure out how to unhook it from the rest of the unit. There is one plastic tab that seems to be holding it to the unit and I am afraid I will break it if I keep trying to pop it out.