忍不住：cannot help to doing sth. Normally there is an action after the person say it. And the action can be negative or positive. For example: 我再也忍不住了！明天就要把真相告诉他！or 我忍不住痛打了他一顿。他这次太过分了！ or 我忍不住给他一个大大的拥抱，他太可爱了。 受不了：cannot stand sth. or sb. There s always a negative emotion when people say that. 用法：受不了 sb./sth. or 受不了do sth. or 受不了sb. do sth. 他受不了老婆的坏脾气。 or我再也受不了穿这种制服上班了！or真受不了你每天睡懒觉！ 舍不得=不舍得：cannot let go of / give up sth or someone. Normally there can be noun or verb after it. And the expression is always positive: to show the love or attachment of sth or someone. 我舍不得离开我的故乡。or 真舍不得你。or 这蛋糕太好吃了，他竟然不舍得分给我一点。

忍不住 means could not resist and 舍不得 means you're reluctant to do something. Examples: 我忍不住了！I can't stand it anymore! 我舍不得让你走... I don't want to let you go.

忍不住=cant help doing sth 我忍不住再吃一口（I cant help having another bite） 舍不得=dont have the heart or unwilling to do sth 我舍不得离开（I am not willing to leave）

他忍不住哭了 我终于忍不住了，大声喊道：“他妈的，这个会议要开24小时吗？” 她实在太漂亮看，你忍不住了，终于犯了人生最大的错误---把她变成了老婆 这么好的玩具，小明舍不得给小西玩。 就要分别了，她有点舍不得

忍不住 mean you want to do something very much , can't help doing it 不舍得 mean you don't want to do it ,for example ,you like a girl, so you 不舍得 hurt her