Unless you are already aware of the damage it causes, you probably say "should" at least a few times a day. According to Merriam-Webster dictionary, the word, should, expresses obligation or expectation. The internal message, however, when we say, "I should" do something, is inherently negative. For example, saying "I should go to the gym" means I know I would benefit from going to the gym. What is implied in the should statement is, "but I really don't want to right now" and "I feel guilty about it." If there wasn't an underlying discord, you would simply say either, I'm going to the gym (because you should and you are) or I'm not going to the gym today (you're not going and you're ok with that choice).
The word "should" indicates that you aren't doing what you would benefit from, and you're not entirely resolute in your choice, either. Enter contrition. Cue the reinforcement of existing negative messages you already hold about yourself. "This is why I'm still fat," "this is why I don't have the body I want," "I'm not good enough." Now you feel bad about yourself for not going to the gym, so you... pour a glass of wine, have a comforting snack and turn on the tv to take your mind off feeling bad about yourself. And tomorrow you will rinse and repeat, if not about going to the gym, about something else. Possibly many things.
Should goes hand in hand with self-reproach. The problem with guilt is that it's largely worthless in terms of getting you to do anything. You may have been raised in a house where guilt and shame was used to get you to do all kinds of everyday things, like be nice to your sister, clean your room and go to church on Sundays. Guilt may feel familiar to you, and humans seek what we know, even if it's bad. But guilt doesn't get you to do the big things like find a job that makes you happy, leave a relationship that sucks the life out of you, or manifest your dreams in love, life and the pursuit of happiness.
How do you free yourself from "should"?
When you hear yourself say "should":
1. Evaluate what you really mean. Do you really mean that you are addressing an obligation or expectation that is worthwhile for you to do right now? Or is this some old mental model you have in your head about you, who you "should" be and what you "should" be doing, that maybe isn't worth holding on to?
2. Do it or drop it. If you evaluate the "should" and decide it's worth keeping, then get up and get going. Drop the "should" and the guilt and just do it. If you evaluate the "should" and you need to let go of the expectation or obligation, then let it go! Really. Stop feeling bad about yourself over it, which, as you know, isn't getting you closer to accomplishing it.
3. Find and replace. Now that you've chosen to do it or drop it, replace "should" permanently with more decisive, positive reinforcers -- words that will serve you better than "should." Try "I accomplished", "I decided," or just simply "I did" or "I didn't." All of those are empowering around an action, whether you do the task or not. Statements like "I will" or "I won't" show you have made a decision and agree with it. This ends the disconnect between where you are and mental images in your head that don't matter.
You control your thoughts. Your thoughts create your emotional state. Your emotions direct your actions. Your actions seek thoughts that reinforce them. It's a circuitous relationship that can be either very positive or very negative and self-deprecating. The important part is you choose it. You have chosen to hold every thought you've ever had, even the ones based on what was role modeled for you. And so you can choose every thought from now on. Negative self-talk and "should" might show up, baggage in hand, but you can show them the mental exit door. Solidifying your decisions and dealing with "should" takes practice, but it's work worth doing. You really should stop saying "should." Better yet, just do it.