Can we learn multiple languages at the same time?

I've learned English for several years since it was mandatory for students. I'm still trying to be better at English but I now I want to learn more languages. I'm interested in Korean and Chinese. I've started learning Korean by myself since April this year. I realized that I've forgot a lot of English words since then. I did not completely forgot the words, they were just somewhere on my mind, showed up and disappeared sometimes. Therefore, my reaction when speaking gets slower. I can see that's a bad sign. I'm forgetting English while focusing on learning Korean. 

So, my question is: Can we learn multiple languages at the same time? And how can we master them all?

My Korean is getting better, but my English is getting worse. *sigh*

Language partners

Hi, I'm a native Vietnamese speaker. I'm currently  learning Korean. I find it really difficult to memorize new vocabularies, also grammar rules. It's would be nice if anyone here is a native Korean can spend sometimes to help me. we can practice English or I can teach you Vietnamese if you're interested. 

감사합니다...

It would have been good... I got invited to an interview for the teaching assistant job at a school at 2 p.m yesterday. To be honest, I didnt think I'm qualified for the job since they required good English skills (the English level of candidates must be around band 6.5 of the IELTS test). But I still sent my CV to them. Surprisingly, they replied back to me 2 hours later asking if I could attend the interview the following day. I was in my hometown at that time. It's 60 kilometers away from Hanoi. I had to wake up at 4 a.m to go to Hanoi and then went back home the same day just for the interview. I got to Hanoi at 6 a.m. I walked to my rent room and slept a little. At 1.30 p.m, I went out to print out my CV and headed to the school. Sitting there at the table were a foreign guy and two Vietnamese girls. I got excited because I've never been interviewed by a foreigner. He started to asked me some questions. He was nice and friendly. The way he talked to me made me comfortable. This interview was beyond my imagination. He only asked me about my hobbies, my hometown and my tutoring job I'm doing at the moment. The two girls were kind, too. They patiently listen to me. It was not an interview, to me, it was actually like a normal conversation between me and my friends. I left there at 3 p.m. I walked to my rent room to get my backpack n jumped on a bus to go home. I was fully satisfied and happy even though I knew I didn't do it good. My answers to his questions were short and lack of information. I also made grammar mistakes and misunderstood him somtimes. But I didnt really mind it. "It will be ok if I fail, I already had a good time talking to them. I'll try to study harder and apply for the job again" , I told myself that. I got home at 7.30 p.m. After dinner, I opened my laptop n read my CV again. That moment I realized I had made a big mistake. In my CV, I wrote that my students I've been tutoring are 6 and 14. The time and dates I mentioned in my CV were not accurate, they were just close to the real dates since I didn't remember exactly. But when he asked me about my experiences, I told him I got two students at the moment, they were 8 and 10. Then he started asking me a bunch of questions about my students, like if they are brothers or sisters, the name of their school, the time I have taught them and some weird questions. It was clearly he doubt me. I was being a liar to them. Thinking about that makes me sick. Everything I told them was the truth. I'm not a liar.

I'm done I wrote over 2000 words in almost 1 hour and just an unexpected click, it disappeared. Well, my life is over. What a bad day already.