I think it is a poor policy to require yourself to understand every theorem before applying it. Imagine you had to understand how every car, fridge or computer worked before using them ... you would be paralyzed by inaction.
Granted, as a mathematician (assuming you are one) the situation is a little different because we are talking about understanding things in your field, but mathematics is extremely vast, and I daresay no one alive today knows it all. This doesn't prevent it from being a deeply interconnected thing, however, such that results from on tendril are often required in others.
My advice is to use any result as necessary, and if it becomes something that you employ often, then it may be a good idea to begin to understand it on a more fundamental level. This depends on things such as how much theory is required to understand the proof of the theorem, how much time/effort that will require, how useful will those efforts be for you in a practical sense.
If it is considered a part of your branch of mathematics, it may be something you should eventually get around to fully imbibing. But you can't do this with everything, and I am sure there are some elementary results whose proofs I've never looked up.
You can easily lose weeks of time reading books and papers in an effort to understand a result, in the end to simply apply it as you would have and not feel like you've gained anything else from it (though sometimes you do). I'd try to avoid that kind of a bitter experience if possible.
But don't feel embarrassed. For one thing, it does you no good, and for another, there is nothing shameful in simply being yet to read a proof. Math is a toolbox, and if you pick up a tool and use it as intended, it doesn't always matter so much that you understand how the tool operates/came about.