Your willingness to hear constructive criticism suggests a desire to grow and learn. Record their observations in your journal so you can consider them in your next search. Then check in after two months or so. Standard procedure is for companies to hire on three months' probation. This is because many new hires do not work out, and the probation period allows the parties to assess each other in practice without commitment.
Rejection is a natural function of job searching.
You shouldn't take it personally, but you should look for clues you can use from each and every turndown. And depending on your circumstances, you may need to rethink your strategy. Talk to advisors about your goals and your options. Show your resume and cover letter to people you respect and admire, and ask them to identify any obvious problems. Ask for suggestions about how suited you are for the positions you're seeking and for ways to strengthen your application.