Here are some day-to-day tips for building a thick skin.
Don't give yourself the time to become invested in a design before you seek feedback, either from colleagues or clients.
Don't stop with one early critique. Ask for feedback at every convenient opportunity.
Create many different ideas, sketches, scenarios, etc. before you start to refine them. Evaluate each one with others, generate a new set, and repeat until you narrow in on an ideal concept which you'll iterate further.
Sketch your ideas with simple but permanent materials. Get the idea out of your brain and move on - don't refine it or make it look pretty (yet). Certainly don't write any code or use Adobe Photoshop.
Sometimes, in acute cases of design ego, it might be easier to hand your materials over to a trusted colleague who will seek feedback on your behalf.
Mentally prepare yourself for criticism. Tell yourself in advance that your design couldn't possibly be perfect and that more eyes on your design will make it better. Remember that your value is not measured by a design, but rather by your ability to engage in the design process. You are a conduit for translating many disparate ideas and suggestions -- some yours, some others' -- into something tangible.
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