In this case it is more of a glass pot than a terrarium. The sun is probably not good for any roots against the glass yet the plant needs full sun. The plant you chose, a jade (Crassula ovate) is fairly easy to grow in good light, but it is a plant that wants good drainage, and it would do better in a pot with drainage holes than in a glass like that. As you already discovered it is difficult to get the right amount of water in a "pot" like that......too much and the roots die off and drown, too little and the plant dries out.

Adding on to what woodnative said, Cassula ovata, or the jade plant, is not a good choice for terrariums (no succulents or cacti are). They all require gritty or very well draining soil. Cactus growers have a little saying about rocks under soil: put a wet sponge on top of some pebbles and what happens? literally nothing. Same thing happens with dirt. I am not a terrarium expert at all, but I do know that cacti and succulents die easily in them.

The other two have hit this nail on the head. I'd have used a cacti mix that drains well. At the very bottom you could layer gravel or perlite so that any excess water drains through to accumulate down there and you can /see/ if you have watered too much. It then can be easily removed by tipping your dish or terrarium slightly to one side and siphoning off excess with a turkey baster. But with any kind of succulent that doesn't like to stay wet, an open dish is better with drainage holes and gritty mix, not potting soil.