You may what to focus on other areas of improvement if you are still learning go. Choosing joseki is hard.
Playing a joseki should never be automatic. It is good that you are trying to do more than just memorize joseki sequences. That often leads to a lot of problems as players blindly follow joseki patters and feel that just because a result is "equal" in a local joseki that it must be good. A joseki is rarely equal and even professionals can argue about whether or not a joseki was correct. Sometimes a pro will spend minutes thinking about their response to an approach move only to play the most common joseki that any 15 kyu player would choose without a second thought, but that time was not wasted because if there is a better joseki to be found it will impact the rest of the game.
All of that said, choosing a joseki is actually a fairly high-level concept. It requires a great deal of knowledge about thickness, territory, and when to tenuki. Many, many joseki have "tenuki variations" where the approach move or a later move is ignored in favor of playing elsewhere. The most famous is probably the taisha - a 3-4 low approach tenuki variation. There are double approach joseki for the 4-4 stone if you tenuki from that.
But, if you want to know, here are some basic things you can consider when choosing a joseki.
The simplest thing to do is visualize how the joseki ends and what that does to the board overall. For example:
Which 3-4 joseki is better? a or b?
Here are the final joseki positions. Assume you are black.
Now, "a" gives more solid points, but "b" gives much more potential toward the top side of the board. In both cases black comes away with sente as white needs to play
out on the left to create a base for his group. While "b" creates more potential, on an open board black needs another move to secure the top. For that reason "b" isn't usually played on an open board. In fact, "b" is rarely played in professional games nowadays even though the sequence is joseki. However, what if the board isn't open?On this board a move at "b" actually looks really good. It gives black a large amount of potential on the top and does so fairly solidly. This joseki is hard for white to counter when the board is like this.
In fact, on this board the joseki "a" may not work because black has too much potential on the top. When choosing a joseki remember that your opponent can also choose their response - they can chose another joseki variation of your joseki choice. If black plays at "a" white might decide to push and threaten a fight to limit black's potential there.
After defending, black then has even more joseki choices because this joseki is a complicated one. I cannot get into listing all the possible outcomes but this is the standard sequence before black has to really choose his next move.
So when you are choosing a joseki you need to try to visualize the board and how it will look not just from the list of joseki you know, but from a large list of potential joseki. For example, the last joseki I showed is the start of what are called "avalanche variations." From there are hundreds of possible joseki sequences including many "fighting joseki" that leave behind floating or unsettled groups. I can almost guarantee you that no one has them all committed to memory. When a professional game involves one of these variations the players aren't trying to remember joseki, and they never stop thinking. They are reading out the fights and the board position to choose each move. There are always more variations to a joseki and many times a move that looks questionable can still be joseki.
Even worse, though, is when your opponent plays a "noseki" sequence. This refers to playing a move that isn't part of a joseki and that is a mistake. Even moves that are not joseki can be good, but even if the move is bad it is always possible that your response will make it okay - countering their mistake with one of your own. If you only memorize joseki and don't learn why the moves are the way they are you are very likely to make a mistakes when your opponent makes one of their own.
For these reasons it usually isn't good to memorize more than a few dozen joseki. Instead, try to learn why the moves in a joseki sequence are considered good and add that reasoning to your play. Always be thinking when playing each move in joseki because there could be a better move. Good players are able to play joseki moves without memorizing the sequence because they understand the fundamentals of go well enough and pros are always changing what joseki they use in games.
For example, this is probably the most common 4-4 joseki:
However, many pro players are now choosing not to play
anymore because they feel that defending the corner is too slow for black. Instead, many of them either pincer white or tenuki now after 
.So just like the "b" 3-4 joseki has fallen out of favor in recent years, the most common 4-4 joseki might also become "old" if this trend continues. Still, it shows an important point: pros don't just play joseki. They choose their joseki at every stage from when they start it to when they finish it. And they are always looking for the best move on the board - not just trying to play joseki.
Choosing joseki is hard.
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