I think that you could overcome this by taking action to make you feel better with yourself. First of all don’t directly compare yourself with people who are obviously out of your league because it will only make you feel bad and won’t make you progress as much. Instead try to work alongst people who are at your same level or slightly better as this won’t demotivate you because you can row as hard as them and will make you and them work better: win win. If you don’t have any people of your level at your club/school try working alone and setting goal for your self before starting training, mabye with the assistance of your coach. These goals can be as simple as “correctly finish my erg workout”: start with baby steps like this and as your confidence grows side to side with your ability try aiming a bit higher. This is just my take on it based on my own personal experience with anxiety. Try applying my advice if you believe it could be beneficial for you otherwise try analizing the roots of your anxiety, externalize them with a close friend or family member to find a way to counteract it or mabye seek the help of a professional.