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I recently had a conversation with a friend who wanted to go back to school to get a college degree. He currently had a decent job, but wanted more opportunities. I told him he was in a great place to go back to school because many colleges are offering online classes and evening courses so he would be able to get his degree without disrupting his income stream.

“Oh no,” he told me. “I can’t work and go to school at the same time.” I assured him that he could and while it might be hard, it would be much better than going without an income for a few years. He was convinced he couldn’t do that and a few weeks later I found out that he had managed to get himself “let go” from his job which meant he could collect unemployment for a short period while getting back into college.

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I was disappointed. I thought, this guy has some real potential, but he’s never going to reach it unless he can push himself well beyond what he thinks he is capable of today. He had convinced himself that what I was suggesting was way too much for him to handle. This is what we call the “glass ceiling,” and everyone has one.

Usually a “glass ceiling” refers to things that externally keep you from reaching your full potential, but for most people, the thing holding them back is themselves. What I was suggesting may have been so far beyond this guy’s self-imposed glass ceiling that he wasn’t even willing to think about it.

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But why? What I was suggesting wasn’t particularly difficult and I’m saying that having had the experience of doing a two-year master’s program in nine months while holding down a demanding full-time job. The difference is that I had made a lot of conscious decisions to push myself to (and sometimes beyond) my capabilities before getting to that point. My self-imposed glass ceiling was a lot higher because I had spent a lot of time banging my head on it and, as a result, pushing it higher.

My friend has always avoided running into his ceiling, so when he was faced with a challenge, that would mean pushing it up several levels, and he just couldn’t do it. He hadn’t invested the time and energy ahead of time in pushing it up to the point where he knew he could do it based on his previous achievements.

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When I’m doing something really hard, I often find myself thinking back and saying, “But this isn’t much harder than that thing I did last year, so surely I can do this.” If you don’t have those moments to look back on, it is difficult to really achieve your potential because you don’t have any frame of reference for your real capabilities.