Congrats on losing 20kg, that's awesome!
Perhaps you just need to shake things up a little. Have you tried varying your exercise routine? Maybe up the weights you're lifting (or if you haven't been lifting any, start), change a steady run to a mix up of run and sprints, try a different sport....most importantly, make it something that will be fun and that will allow you to enjoy the challenge. If you hate it you probably won't stay on track!
If the change in exercise doesn't seem to have make a difference (don't judge this by weight alone...take measurements) then you probably also need to look at dropping your calories a little. Though, like Cantona said, not if the problem is that you've already been eating too little.