Welcome to the fraternity. Being dropped by another rider can be inspiring, humbling, humiliating, and all of the above at the same time.
You don't know if the guy on the MTB was a local racer doing something different, an MTB'r doing intervals, someone who's been commuting for 20 years and has legs of iron, or just someone who's faster than you.
Going faster on a bike is a goal, but there are other fun things to do on/with a bike. Figure out what YOU enjoy doing. Then set some goals, doesn't really matter what they are, but write them down and post it someplace where you'll see it daily (sounds corny but it works).
A reasonable goal at this stage may be to pick a local charity ride/distance event (there are plenty that don't have a big pledge amount) and focus your training on completing the event. Then pick the next one with the goal of bettering your time on the first one. Another goal is to start participating in a regular group ride (many local clubs and bike shops run these). Start riding with one of the slower groups, see how you do. Then set a goal of being able to ride comfortably with the next fastest group by the end of the season. Or you may discover that some other aspect of cycling tickles your fancy - could be long distance rides (radoneuring), or touring. Whatever it is, this will give you a focus to your riding beyond just getting faster. It'll be getting faster with a purpose [ride 100 miles in under 6 hrs, ride 200 km, ride across the country, or bragging rights on the local club ride].
Once you have some goals, there are a wealth of training plans and suggestions out there on how to meet those goals better and still have fun while doing it. Plenty of links in these forums and on the net. 'Cause without fun, you'll start asking yourself (or your SO will start asking YOU) why you're killing yourself on the d*mn bike when you could be doing something less stressful. [As someone a bit older, my response is biking is healthier than golf, cheaper than a sports car, and safer than a mistress!
]