Factory Reset has solved my problem. This also solved my second problem which was automatically turning off sometimes and then showing 'Starting apps' and taking too much time on it.
-> First of all I saved all my data to sd card and then switched off the phone and took out my sd card.
-> Then I reached to options by pressing power button and volume up button and releasing power button when logo is shown while continuing pressing volume up button.
-> Then I selected wipe data/factory reset option. It took just a few seconds to complete factory reset and then I rebooted the phone.
Reference : https://drfone.wondershare.com/reset-android/wipe-data-factory-reset.html