Uneven surface painting

[Bushbaby]

Hi Looking for some advice please. Trying to paint my daughters room and struggle with getting an even, smooth finish on the plasterboard. Think I may have stupidly sanded it in the past for paint adhesion and its patchy in places which I can only describe as appearing like wall acne. I put about 7 coats of paint on and the blotches just keep coming through. It was Wickes paint I used. Any advice on how to cover this for a smooth even finish please. I was thinking along the lines of a very fine skim but not sure.