Back Camera Works on My iPhone 6 but Front Caera Sometimes Doesn't Work or Freezes

I have a prblem with my iPhone 6 for a long time now and I need a solution. The rear camera works fine (always) but the front camera most time doesn't work. When I switch from a photo to a selfie it freezes for a second and then goes back to normal. I went into a repair store for a screen replacement and asked for the camera to be fixed as well. I wasn't charged for the camera fix, but the repair store guy told me that the camera works fine,and it was, but only for a few hours. Any idea what may cause this? Should I just replace my iPhone altogether since it is an older iPhone 6?