I had the same feeling with the americans, but I got used to it. Sometimes they talk very fast and they have different ways of speaking according to their social group. In the other hand, I found other native english speakers mich easier to understand. For example the South Africans. From my experience, I noticed that they usualy speak english very clearly.

I think the best thing to progressively master the oral english is to start practicing it with people that have it as a second language. When I started practicing english, it was mainly with taiwanese people, chinese and indians (northern indians, I still can't understand the southern ones, their accent is too strong). You can also try with the europeans. The dutch, the scandinavians or the germans. i think their english is globaly easier to understand.

At the same time, you need to get to watch english tv series (Breaking bad is a really good one to get used to the slang language. Jesse Pinkman starts all his senteces with Yo ! And says Bitch ! 3 times a minute ). You'll progressively be able to understznd this kind of speech like in your video.

I think as you practice listening more, you will eventually be able to understand her.  I don't know if this happens in other languages, but in English, native speakers in casual settings will slur their words together.  To understand, and to fill in what you didn't understand, you need to be very familiar with common phrases and expressions (including slang and curse words) in English.  I watched the whole video, and her pronunciation and way of speaking are pretty average for an American, even though they're not necessarily proper.  The guy interviewing her was a bit harder to understand.  He sounded almost like he had a mouth full of food.  Unfortunately, since I'm a native speaker of English, I can't tell you how long it will take for you to be able to understand her, but I believe you can do it if you work hard!

YouTube is a great place to find examples of Americans speaking casually, the way Rhonda was in this video.  If you want some video recommendations, you can message me (I spend way too much time on YouTube).

Rene, not understanding such videos does NOT mean that your English is very bad. Understanding one on one conversation is much easier than understanding videos and that's what's really important. When I watch movies in English I even use subtitles because I don't understand everything. Take into consideration different accents and lack of context and plain old mumbling and it can be hard. If you are in a real conversation people will watch you for clues that you understand and you can ask questions. Don't be too hard on yourself! 

Oh and she was using slang in her interview The way she talks its like she is saying the words longer than usual but it isnt too fast Keep repeating the video and you'll pick it up. Oh and Rousey is a beast!!!!  Good taste in a video!

I'll admit that I didn't listen to the entire video but I definitely missed the part where the guy is fucking off the the bear cub.  There were some slang words in there but the overall language seemed easy to follow, slowly spoken.  Poor bear though!