There can be many different reason for someone to leave her or his job. You will need to think about resigning from your job more than accepting a job as resigning might be irreversible.
Resignletter.org has an example for any reason. They are free and fully editable letters. All you need to do is to customise it before sending it to the employers.
Leaving your job shouldn’t be too difficult. If you feel too difficult to leave then maybe you shouldn’t leave!
Usually, most employers wouldn’t question and accept your resignation straight away as they have many others who are waiting to be employed with the company that you are about to leave.
There are a few companies who are offering exit interviews to understand why you are leaving them but don’t take this wrong. It’s not because they will beg you to stay. They will just to see what you have got to say and if they could improve anything at the workplace.