David45, I agree with hardloper: "just run baby" and with Swim Bike Run: "you just need to run, dude".
Case study: A person very close to me recorded HRs of 220 bpm during hard workouts when he was in his early 30s. His running buddies gave him plenty of good-natured abuse as a result. He checked with his doctor because he was concerned. The doctor did an EKG test, and said all was fine. Almost 30 years later, he is still reaching HRs higher than what the equations predict, and he is still taking sh*t from his running buddies.
Heart rate (HR) of 198 is not a problem in an 18 year old, as long as you don't have one of the rare conditions that increase your risk of a cardiac electrical malfunction. See your care provider if you are truly concerned. Those conditions are sufficiently rare that widespread screening for them is not considered a good use of medical resources.
The old equation for max HR (Max HR = 220-Age) is from Fox, Naughton, & Haskell (1971, hard to find). More recent and better studies have found somewhat different equations. A HR of 198 is OK in a healthy young person.
Individuals vary. For example, Gulati et al. (2010) report a standard deviation of 12 beats per minute (bpm) about the predicted line for max HR vs. age, which means 32% of subjects have a max HR at least 12 bpm lower or higher than the HR predicted by the equation.
Multiple studies, such as Tanaka, Monahan, & Seals (2001), have shown that max HR in an individual is not correlated with fitness, but resting HR is (negatively) correlated with fitness.