Sharepoint - "Sorry, this site hasn't been shared with you" when trying to access MySite

Try creating a permission policy for your web application on Central Administration followingly:

1. On Central Administration, navigate to Application Management -> Under Web Applications: Manage web applications
2. Select your My Site -web application by clicking it, and open Permission Policy
3. Click Add Permission Policy Level
4. Enter at least the Name for your Permission Policy
5. Select following Permissions:
6. Create Subsites - Create subsites such as team sites, Meeting Workspace sites, and Document Workspace sites.
7. View Pages - View pages in a Web site.
8. Browse User Information - View information about users of the Web site.
9. Open - Allows users to open a Web site, list, or folder in order to access items inside that container.
10. Leave other parts unmarket and click Save
11. Make sure your My Site -web application is selected and open User Policy
12. Click Add Users, and on first view leave Zones: (All zones) selected, then click Next >
13. On Choose Users -part click Browse (the catalogue type of icon). Click All Users and select Everyone activated. Press Add -> and OK
14. On Choose Permissions, select the Permission Policy you recently created, and finally click Finish

In case you haven't already followed the TechNet's guide Configure My Sites in SharePoint Server 2013, you should from now on :)

Had the same problem, found this here (worked for me):

If you have credential for account who has rights on site then, by simply hitting below URL, you can have prompt to login prompt.

http:/serverurl/_layouts/closeConnection.aspx?loginasanotheruser=true