I am so sorry that you have been diagnosed but wanted to say welcome to the forum . Your English is almost word perfect - in fact if you hadn’t mentioned that it wasn’t your first language I would never have guessed.
MS is believed to be an auto immune disorder - that is, your immune system attacks your central nervous system and damages the protective sheath that protects the nerves which is why symptoms can appear anywhere in your body at any time. The Avonex injections (and there are other similar injections) are designed to dampen down your immune system to prevent your body attacking itself. The Avonex should reduce the number of relapses you have (attacks) and the severity of them (that is, make them much milder).
I take a similar drug which I inject three times a week and I believe it is working for me. I had three relapses in the first year and then, shortly after starting on the injections, went for three and a half years without a relapse. I have just had another relapse but it was much milder than previous ones.
Many of us do still have symptoms between relapses and there are treatments available for this so it is always asking your nurse or GP about these. It sometimes takes a bit of trial and error to find the right medication that works for you so if the first one doesn’t suit you, go back and ask to try another one.
Lastly, you are so young to be diagnosed but I want to reassure you that most of us with MS still enjoy life to the full. I have had MS for 5 years, I am 48 and I still work, still drive, still party, socialise, take holidays etc. There are challenges along the way, but most things are still possible.
Best wishes and good luck for your future studies, dating, marriage and whatever else your dreams are. Go for it