You'll continue to feel this way in first year (for how long varies wildly). When you look at your first exam grades you might doubt yourself even if you got "passing" scores. The key is to not let the doubt get the better of you. Channel it into productivity. Analyze your personal habits and study routines as objectively as possible and make adjustments. Also, make sure you don't go through it alone. Whether it's classmates, family, outside friends etc, have a support system you can use when you need to vent (you will). If you find a group of students that you enjoy being around and that you work well with it gets a lot easier IMO.
For now, make sure your personal habits are conducive to the medical school environment. If they aren't (you miss alarms in the morning/are a night person, you don't exercise enough, you eat irregularly or have a poor diet, you have personal relationship issues that may get in the way, you have untreated depression/anxiety that tends to rear its ugly head at the worst times, etc), try to correct them before you start. Also, if you're like me and could cram within 72 hours of a biochem test and come out relatively unscathed, keep an eye towards improving your long-term retention and try to get in the habit of not procrastinating. It's 10x more difficult to deal with this stuff while meeting the demands of medical school.
Good luck everyone, and congrats 🙂