When you stick by somebody you support and help them. You probably know how tricky phrasal verbs can be - ‘to stick by’ means almost the same as ‘to stand by someone’. Both are phrasal verbs.
A more formal expression with similar meanig is ‘to adhere to somebody’ but you have to use the verb ‘adhere’ with the preposition to not by.
I know that they have all abandoned you one by one but I hope you won’t forget I’ve stuck by you from the beginning.[YSaerTTEW443543]