Understand what you can do. Try new things and do your best at them, but if you find after many tries that it is something you can't do, throw in the towel and be realistic.
Explain to others that you are being realistic in your approach. When you hit a wall, back off. You can't continue to bang your head against the wall. Everyone has limitations. Some people don't accept them--but they are there.
Challenge yourself. You need to try before you can be realistic and say, "I can't do that." No one ever got anywhere without trying. Many things can be accomplished in different ways. Sometimes you need to find the one that works for you.
Follow what you believe. People may tell you that you can do something if you try hard enough--and sometimes they are right--but if you know that you can't do it, follow what you believe.
See what is there. You don't need to analyze everything. You can be realistic and just go with the flow. Don't look below the surface; just take everything at face value.
Approach life realistically. Always expect what you know to be realistic. Don't look for miracles. Just accept life for what it is--life happens--accept that and be realistic.