Could using a showerhead minimize your chances of orgasming with a man?!

Short answer: no.

You can't damage or de-sensitize your clitoris through masturbation (even with a shower head). If anything, you will find the opposite happens: most women can orgasm more quickly and easily if they masturbate regularly. The clitoris gets accustomed to being stimulated and is far more likely to maintain arousal.

However, eventually everyone gets to figure out that not all masturbation sessions are the same, and not all orgasms are the same. Sometimes they are very intense and sometimes they're not. Stress, tiredness and anticipation can have a big effect on your orgasms. If we have a great orgasm one time, it can often be difficult to duplicate that the next time, even if we try all the same things. Some orgasms are just better than others.

One thing you probably will have to be aware of is that intercourse (when you eventually get there) is not likely to be a magic source of orgasms anyway. Any sexual partner will need time and patience to get to know your body and what you like (and you will find the same as you get to know them). Most women also require clitoral stimulation in order to bring on an orgasm - which intercourse rarely provides. However, there are plenty of different means to bring a woman to orgasm, as you're already starting to discover. Don't be too embarrassed by the suggestion, but if you don't enjoy direct rubbing or vibration then you may find that you respond better to oral sex later in life. It's usually a lot gentler, and there's no vibration involved, so it will tend to mimic your showerhead a bit more closely.

Well, if all else fails during 'the real thing', just visualize your shower head at the appropriate moment.

No.

First of all I think it's awesome that you know you aren't ready for sex at your age. And masturbation is the healthy way to explore. I wouldn't worry about damaging nerves unless you are beating the hell out of your clit. Chances are you aren't! Orgasms are powerful and very intense (and fun!)
Once you are ready for a partner, don't get bummed if he can't compete with the shower head...you may not orgasm with intercourse, and that's not abnormal. The good thing is that you will already know what works for you and you can help him help you when the time comes.
Just respect your body and know that you're not doing anything that any other 14 year old hasn't done for centuries! Have fun :)