I too am a self confessed 19-year-old thumb sucker and have tried many things to make me stop.Some suggestions I have are: Put nail varnish on your thumb (you can even get special ones specific to stop thumb sucking!), tastes horrible and it usually stops me for a while. Put a plaster on your thumb, when you go to suck it, the texture will feel weird in your mouth and it goes soggy on your tounge, which will probably put you off. (Then again..whenever I try this I usually end up ripping the plaster off with my teeth!) Whenever you get the urge to suck your thumb, try chewing gum or sucking a lollipop, this will hopefully help to keep your brain and mouth occupied by doing a different action in place of sucking your thumb. Or similarly, do something with your hands to take your mind off it.
Another thing I have tried is to get someone to tap/flick you gently whenever they catch you sucking your thumb. Eventually it is meant to make you associate pain/punishment with sucking your thumb which will make you stop. Discipline yourself so that you allocate specific times where you allow yourself to suck your thumb, like a night time or when you're tired for example, but stick to it. Hope some of these suggestions are helpful! Good luck :)