Creating your own encryption and decryption algorithm I'm very uneducated when it comes to cryptography. I have tried to find an answer to my question, but what I've read doesn't quite cover what I'm asking.

I have thought up my own encryption algorithm (which I'm sure is nothing new) and I would like to know why this sort of algorithm is not used in terms of security, efficiency and any other reasons.

It stems from the Caesar Cipher. But, instead of shifting each letter by 13, you shift each character (using computer character values) by a certain value  each time.

So, let's say I had the sentence:

My dog's name is Rover.


And I have an array of numbers:

1, 4, 3, 2


I would shift M up by 1 character, y by 4, [space] by 3, d by 2 and then continue with the rest of the sentence, looping through the number array.
So, the sentence would be converted into N}#fpk*u!rdof$lu!Vrxfv1.

Let's call the array your "key". Only someone with the key could decipher the message, unless someone figures it out. The more items in your array (key), the safer you are from someone deciphering your message.

Apologies if my explanation is poor.
To summarize my question:


Why is this bad (or, not used)
Is it inefficient/insecure compared to other encryption methods
Is there anything else about this that should be considered


Thanks.
Edit: how to break Vigenère
Yes, this cipher (which as SEJPM points out, is Vigenère) is vulnerable to frequency analysis, but Vigenère resists this a bit, because common letters don't always get ciphered into the same ciphertext. The unavoidable vulnerability stems from the repeating nature of the key.
Let's say your message is $N$ characters long and your key is $k$ characters long. You may think that by repeating the key until you reach $N$ characters that you gain some &quot;units of security&quot; (&quot;it's $\lfloor N/k \rfloor$ times more secure!&quot;). But if I know your message is enciphered in this way, I have learned a great deal about your plaintext.
If I can estimate your key length $k$ $-$ and I can, with the great Kasiski's help $-$ then I can take every $k$th ciphertext character and group them together (for example, if $k=5$, group together the first, sixth, eleventh, etc. characters, then group the second, seventh, twelfth, etc. into another block, and so on). Each block is enciphered with the same character, a character from your key. At this point, a straightforward frequency analysis comes into play; breaking this is no harder than breaking the Caesar cipher you started with.
I do this for each &quot;group&quot; and now I have recovered your key.

It is cool that you came up with the idea of a polyalphabetic cipher on your own. But as the comment and other answer have pointed out, this cipher is insecure and can be broken quite reliably. Seriously: only use this for fun or pedagogical reasons.
In 1863, Kasiski even knew how to guess the length of the key for this cipher! That forces you to make your key longer / harder to remember.
It's very efficient; you can encipher text quickly with it. But that's a double-edged sword: I can attempt to decipher it quickly as well.