I can't make any claims as to what is normal or not, but I can say I have observed other players who tend to "role-play themselves" or to role-play very similar characters who are not themselves, so it seems not uncommon. I can also say that as the years have gone by, I have found myself creating and playing characters farther and farther away from my own personality, partly out of greater confidence, partly out of boredom.
Here I pause to make a related observation about character-building as context: I have observed some players to develop mostly fully realized characters/characterizations from the start of the game, and these players often focus on character back-history; I have observed other players who really seem to need three (or five, or seven) sessions to figure out what works, and these players often ignore or react adversely to character back-history. These categories are not absolute, and not every player will fall neatly into line, but I have had some success in understanding individual players in my groups. (I am strongly one of the former, by the way.)
With that context, I offer some suggestions:
FIRST, if you are having trouble role-playing someone who is not yourself, I would not try moving too far, too fast. Instead, I would try to pick one trait, or make one significant difference between your character and yourself. I would try to make the difference large, but not exaggerated to a cartoonish level. I would make the difference something that is mostly value-neutral, or something you can live with playing on a regular basis.
Some related ways to achieve this are:
If this is a game with character stats, like D&D, aim for a character with at least one stat very different from your image of yourself-- for instance, a character who is immensely strong-- and then as the game goes by, take every opportunity to think about how someone who has always been that strong might tend to approach things. (Doesn't have to be that-- it could be the opposite, or it could be a social attribute like charisma, or a mental attribute like intelligence, or even something like background wealth, depending on the system.)
If the idea of character histories resonates with you, consider a character whose background deviates from yours in some large important way, even aside from the difference between this world and the game world. It could be something as simple as a change in wealth level (so this overlaps with the previous idea, a bit) or it could be some major, even some traumatizing event in the character's past, then try to think about how this event would shape someone's outlook differently from your own.
If the idea of a detailed starting history does not resonate with you, consider a trait that you just find interesting-- it might be a positive trait you'd like to emulate, it might be a negative trait you'd like to understand-- and go with it as much as possible. Use the opportunities to think about why a person would act that way, either reflexively positive or negative as you've defined it, and what made them that way. This is almost the opposite of the previous idea.
Try to use the differences between the real world and the game world as a focus. Consider a sort of standard generic fantasy world-- magic, gods, orcs dragons, all that. Surely your character must have a different idea about magic fireballs or healing potions or centaurs than you do. Talk to the GM, see what will be a main theme in the game that is different from ours, and use that as a lever to consistently react differently than you would.
Again, you don't have to do all of these things if you are aiming for a One Significant Difference sort of a character. In fact, you should not do all of these things, and the second and third points are unlikely (in my experience) to both be useful for the same player, especially a new player. These are just ideas for how to approach the issue, how to single out and focus on one significant difference, for one game.
SECOND, if for some reason you really do want to make a radical break all at once and throw caution to wind and play someone very different from yourself, you can try doing... more of the above, for lack of a better description. Make a character with two or more major differences in abilities from you; make a character with a very detailed history; focus on many difference between the game world and this one; consider many different traits; or mix and match from that list.
But I think that might be daunting all at once. So here is a last suggestion, to be taken with a grain of salt, and executed with care and some advice from the GM: Consider a fictional character (or perhaps an historical character) that you like reading about and consider playing a modified version of that character. The good thing about this is that you already have a template of the character in your mind, and you like the character.
The bad things about this idea are considerable, though:
First and foremost, the character you choose might just be a terrible fit for the game so talk to the GM about it! (I tried to run a fantasy/mystery/crime-solving game once, and a player had just seen the movie The Punisher circa 1989 and was obsessing over it. No one had fun. No one at all.)
Second, if other players figure out who you're going for, the character might be seen as a little predictable, and other players might decide to have a little fun with it at your expense.
But I have seen players get good mileage by taking inspiration from an established character before, so I mention it here as well.