Hate to say this because I know it will irritate you, but sometimes women change their mind about this after they do it; this is why doctors won't usually do such a permanent surgery for women under a certain age. I think my sister had her tubes tied in her 20s, but know it was not in her early 20s. (Then later [her second marriage] she married a guy with two kids, so she didn't avoid parenthood after all! lol)
Anyway, recent biological studies say that the emotional parts of our brains aren't done developing and changing when we are teenagers, we are still coming to complete maturity until at least our early 20s. This is probably why many doctors just intuitively reject the notion of sterilizing someone at 18. Stick with an IUD and/or the Pill, and get with a partner who has had a vasectomy. You'll be OK until you can get the surgery.
I am one who can honestly say changed her mind. At 18/19, I swore I'd never have kids. SWORE! I began to have a change of heart around 25. By 28, when I got married, it was all I could think about. It took 10 YEARS for me to realize children were definitely, and irrefutably, something I wanted.
In between these years, I tried different BCs. I tried the patch (annoying), tried the depo shot (great, but the 3 months injection was too infrequent to remember well), and finally settled on the IUD. It was perfect. Easy in, no periods, no worries. Until my husband and I made up our minds to have children and took it out, there was never a thought to it.
Bonus- the Mirena is good for up to 5 years! Then change it out for a new one if you still aren't married and still don't want kids. Then you're good again until you're 30, at whoch point you can make that medical choice. Not too bad to only have to deal with it 2x in 10 yrs. =)