Creating your own encryption and decryption algorithm
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.