A clear coat can be applied right over your existing paint. Make sure it water based and preferably 100% acrylic. Deft is one brand that is reasonably priced and available as a satin sheen. There are several on the market, check with your local paint stores for other options.
This will go a long way as a clear coat over your paint, you won't need as much as regular paint.
The only drawback is touching up the paint (if needed); you would need to also touch up the clear coat or have a flat spot. Also, it might be a little difficult to get a perfect looking wall but you already went the difficulty of applying a dark flat paint so you have the skills to do this with few problems.