How do I get Sheetrock smooth enough to paint with semi-gloss paint

You would need to bring your drywall to what's called in the industry a "level 5" finish.  Basically it's similar to what has already been done to your walls but a skim coat of drywall compound is applied to the entire wall surface.  

Unfortunately it's really not a DIY project and you'd save a lot of money by using a lower sheen paint.

My question would be why are you using semi-gloss for general household applications? 

Today's modern paints will give similar performance as to washability and moisture resistance at a much lower sheen.  Behr's Ultra can be used in as low as an eggshell sheen in even a bath that is being steamed up. It's "nano technology" gives it a very dense, scrubable finish. I personally have the eggshell in both my baths and laundry and it has held up wonderfully. Ultra is intrinsically mildew resistant, although, like all Behr paints, it has a chemical mildecide too.

The point is, high gloss highlights every imperfection that a wall has. If the initial houseframing and drywall job was not good, you might not be able to achieve that "class 5" finish even after big money expenditures.

Simply a really good sanding job, good surface preparation and a good primer and low sheen paint can work wonders.

Also, evaluate your lighting. Very few drywall jobs can stand a highly directional light source shining down the wall! I had a bow in my living room ceiling that looked terrible everytime a ceiling fixture was turned on and shone a light across the ceiling. Simply changing the fixture out to one that only downlighted completely hid the problem.

Similarly, during my contracting days, I would ask the customer to put a Post-It on any surface irregularities that bugged them at night.  The normal daylight would often make such areas completely imperceptible.

Hope this has helped in finding alternatives for your problem.