Hey hey! I had/have vaginismus too.
I personally struggled with tampons for years until I finally understood my body enough at 16 to get one in. Before that I used to clamp up too much to get it in.
Once I started having sex, I could get fingers and tampons in, but it took months to get a dude in there, because somehow it was just different. Coupled with a bad sex life things weren't working great.
I finally scheduled my first gyno appointment when i was getting these severe pelvic cramps that my GP couldn't find the source for. After the gyno gave me a painful examination I burst out crying when she told me there was no physical source of my pain and it was a manifestation of how bad my anxiety was.
After I went on anxiety meds she gave me, I thought, awesome, I'm cured! But nope. Still vaginismus.
I don't know how many times I've cried over how "sexually broken" I thought I was. That no dude would want to commit to me because I can rarely have PIV sex.
I'm 21 as well and I've been sexually active for 3 years now with a couple different partners. I still have vaginismus - my first experience with a diva cup left me naked and sobbing on my bathroom floor covered in menstrual blood after clawing that thing out of me after it suctioned itself against my cervix. Now I can use them just fine.
Chances are this isn't advice you can use, but the #1 thing that helps me have painless, actually pleasurable PIV sex is getting a little drunk or high. It's almost magical how once I have a buzz on I just feel physically and mentally more open. For tampons/cups/whatever, what helped me most was researching what my actual insides look like and what's in there - knowing how my body worked helped me feel more comfortable putting something in there. Meditation and self relaxation skills have gotten to the point if I'm feeling tense and something can't get in there, I can calm myself and it'll go right in.
Things still aren't perfect, but I've realized they don't have to be for me to be happy with my body! My vaginismus is not tied to my self worth - I'm not a walking vagina! I'm worth more than how far I can put something into my twat.
I highly recommend you get screened for anxiety and go on medication (although those can have sexual side effects). I also recommend Googling the hell out of vaginismus success stories whenever it gets you down.
Some day in the future every vagina owner will get a How To Train Your Pussy book at birth, but until then just know that when our bodies misbehave you're not broken.