How can you be 100% brutally honest with your boyfriend or girlfriend?

How do you do it? How do you avoiding hiding ANYTHING and NEVER lying for ANY reason? I'm an extremely honest person already, but I don't always tell my boyfriend everything that I'm feeling or rather in the past I haven't, and he's done the same, but we are both gonna start cause we just feel that you should be able to be 100% honest with your SO about EVERYTHING. So any tips to help me/us out?
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Thanks for your answers everyone! We ended up breaking up cause of some pretty big differences of opinion and how we felt on some pretty major subjects, but it was a good experience and I learned a lot fron it so I don't regret it, I did find put he seems to have a chronic lying problem at times, but over all I still feel he's a good guy and I hope in time we can rebuild our friendship cause we were friends before we got together, but for now he decided he couldn't handle that so y'all pray 4 us
How can you be 100% brutally honest with your boyfriend or girlfriend?
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