Others have already given most of the advice I can think of, but there are two things I would like to add. Most advances in life, with skills, and with personal achievements, all begin with the same thing: tilting your lance and charging at an objective.

Start small with this... one small goal per day. Perhaps it is just saying "Good morning." to 3 people on the walk to your office, or making eye contact with 3 fellow drivers during your commute.

After a couple of weeks, ramp this up to an additional medium sized goal per week. This might be a project at work or home you've been avoiding, or a fitness goal such as jogging 1.5 miles non stop.

After a month or two of this keep it going, but add on one big goal per quarter. This could be taking on or leading a big project at work, participating in a 5k fun run, home renovations, etc.

By doing these things, over time you will build confidence, meet new people, and have fresh, new experiences. Be sure to reward yourself along the way, but keep it proportional (for example, achieving small daily goals may rate a weekly lunch at your favorite place, or watching a movie that you've been meaning to see).

The second thing I would like to add is a companion note to my ideas above, as everything we humans do involves tools and gear. The principle of my idea is, "If I take care of my gear, my gear will take care of me." Whether it is the clothing we wear to work, or to the store, or the car we drive, or sporting equipment, etc., items are more presentable, more reliable, and much less dramatic when they are maintained. Your clothing should be clean and presentable every day, your car should be as clean as is practical (for example, having a dusty and dingy interior is not a happy place), and you should not be mistaken for a homeless person when you participate in sports.