Over criticising your work will only hinder your progress.
A negative thought can ruin the whole drawing process. If you are constantly over-critiquing yourself, you will have a hard time getting anything accomplished, both artistically and in life. A constant self-barrage about how much the sketch isn't turning out as good as planned will not help in the long run. Instead be open to comments and criticism, even your own, but realise mistakes are a part of the learning process and you need to make them, even if that means your first few drawing aren't as good as you were anticipating.
Dealing with frustration.
There are some feelings during the art process that are ok. One is frustration. Frustration just means you are learning. Once you get over that hump, a real break through will happen. You will be able to progress. This is perhaps the single most important emotion that comes from trying to do anything, including art, and the one most people are the most scared of and shy away from. As soon as people feel frustration, they want to stop or give up. Don't do it! Keep practising! It's really tough to overcome, but the rewards will be just as great as the frustration was, if not even more.
Why are you drawing?
You have to know why you are drawing. Is the purpose to practice, for fun, an art final, a commission? If you are overly harsh about just sketching, then you little get little out of it and will be very unproductive. Many people stop drawing as teens due to their drawings "not looking right". They didn't have the right mindset about drawing or got frustrated and gave up instead of persevering . Think back to when you were a small child. Drawing was very fun back then because you just had the sheer desire to create. You didn't care (or perceive at the time) if the drawing of a dog under the tree had correct proportions, anatomy, light and value. You were satisfied it took up a whole sheet of paper and you got to use finger paints.
When sketching, your mind must be open to mistakes. Draw with a pen or just don't use an eraser. Pure sketching involves sculpting on paper. That means that you create the shapes in a way that builds up form and function . At this stage, you can make mistakes, you can experiment. Things don't always look "right". In fact, most things look wrong- especially at first. Of course, that's where the old cliché comes in: Practice makes perfect. In the case of sketching, it may not end up perfect, but you will be able to memorise shapes and forms easier the more you draw them.
For school or a job...
When working on a more professional piece, or a piece that is going to be graded, a different mindset must take place. It's important to be critical of your work, however, over-criticism will hurt you here as well. It's probably best in many instances to get a different sheet of paper(or new layer) and enter the pure sketching phase, then move on with your best sketch to the final product. When moving to the final product, be aware of what you are drawing. You must focus on your end goal. Use decisive strokes, not small timid ones. Be accepting of the fact that you may make a mistake and depending on your medium of choice, that mistake may be more or less forgiving. Understand the limitations of each medium and choose the one that best suits your experience level. Digital mediums have the almost magical "undo button", however drawing on a tablet (or with a mouse!) can be hard for some people to grasp and art programs are generally expensive. Watercolour allows flowing colours and shapes, but if too much paint is applied in one area, it's very hard to take it back.
Reference! Reference! Reference!
Use reference! I can't stress this enough. I've noticed a trend on the internet that many beginning and intermediate artists do not want to use reference. A negative perception has come about because of fears of copyright infringement, theft or worrying about referencing every single source image. I find many of these worries unfounded. If you use only one photo reference and copy straight from it, admitting your reference is wise. However if a drawing was made that included 5 different references, bits and pieces put together to create a new unique artwork, merely admitting that reference was used can be done, but I don't find it necessary. In professional realms, it's understood that almost all completed pieces had, at some point in it's creation, some reference used.
In closing, setting yourself up for a successful or at least fun drawing experience can be as easy as having the right mindset. Don't overly frustrate yourself by making the drawing experience harder than it needs to be, use reference! Remember why you are drawing and make sure your attitude matches accordingly.
***What attitude do you try to have before working on a piece of artwork and does it make for a successful drawing experience?