Fetishism is sexual attraction to an inanimate object or to a body part that isn’t traditionally thought of as sexual. The cause of fetishism is unknown. It can become a problem and be diagnosed as a paraphilia if it interferes with normal sexual or social functioning and if sexual arousal is impossible without the fetish object. Because people who have a fetish are usually very private about it, there is little statistical information about the types or frequency so I can’t tell you if your fetish is “normal”.
What I do know is this: At 14, you are discovering your own sexuality and may be dealing with unwanted or confusing arousal at times. You are young to find a sexual partner, but you are not too young to have sexual desires. It may be that the inside of the mouth reminds you of female genitalia that are forbidden to you due to age or culture. What started as a harmless substitute is now becoming an obsession. That’s just a guess based on little information. I only ask you to think about it.
Nature is unfair. The body is often longing for sexual contact before it is permissible. I hope you are allowing yourself the very normal, usual fantasies about sex with a partner instead of avoiding normal yearnings by channeling them into a fetish that could become a problem for you. Fantasies and self-stimulation are the way that everyone practices getting comfortable with themselves as sexual beings to get ready for sexual intimacy when the right person and the right time come along.
I wish you well.