Before I state my opinion, I want to lead off with a disclaimer. You will most likely not like what I have to say. It may seem harsh. I don't mean to further sour your mood or put you down. This is a game that's meant to be played for fun, but it also has a competitive side to it. So if you just want to have fun, if your goal isn't self improvement read no further. Have a good time, make some friends and enjoy the experience as best you can.
Ok. The truth is you are pretty misinformed. Your recent win rate is not the 10%-20% you claim it to be, nor the 25% your title suggests. I'm looking at your hotslogs right now and over the last 254 HL matches your win rate is 44.9%. While over your last 259 QM matches your win rate is 43.6%. These numbers are a far cry from your claims and they would indicate that you are matched pretty fairly, but your MMR has yet to bottom out. Basically you are still being matched with people who may be slightly better than you, but since you've played so many games your MMR losses and gains are incremental.
I've also looked a few of your last matches. For the most part the teams are evenly balanced. Both in terms of average team MMR and the MMR of individual players. There are a few outliers here and there where you see some players with significantly higher MMRs, but I suspect these guys are grouped with somebody on your team. The MMR is kind of handcuffed in this situation when two players of vastly different skill levels are partied together and searching for a game.
You can blame your win rate on the matchmaker, but the truth is the only person you can hold responsible for your current status in game is yourself. The matchmaker sucks because from time to time it will throw some very strange games out there. You'll get strange comps, sometimes too much variance in the MMR of teams and individual players. In my opinion this occurrence is rare enough that over hundreds and thousands of games played it pretty much evens out.
Again, for an individual match there can be many reasons why you lose. You can get handed a poor comp. The game is not even on skill, so on and so forth. These are things you cannot control. The only things you have control over are your attitude and your performance. If you personally improve your play you will realize that while you can't carry a game for your team, you can consistently make the right play and right decision therefore increasing your chances of winning games. This will in turn translate into an improved win rate in the long term.
Your only choice here is to improve. First I will say you stop playing against AI. This is only hindering your progress. AI are spectacularly easy to beat. They have no coordination, landing skill shots on them is a breeze and they have zero adaptation or game intuition. Anything you learn there is actively making it harder for you to perform against a real human. Look I leveled a few smurfs to get myself a vulture mount and did this by playing against AI. When I got back to my main account for the first few games I was struggling to perform up to my usual standards.
Secondly I suggest you pick a handful of heroes and get really good with them. And I mean really understand them. Most of the heroes in this game can work with multiple builds. Even if they rely on one specific built, there can always be little variations within it depending on opponents and allies. You should understand when these variations are viable and why. If you don't understand what I'm talking about ask and I'd be happy to provide an example.
I would highly recommend that these few heroes be some of the more powerful ones in the current meta. Many of the strongest heroes are quite reasonably priced. All of the 2k gold heroes are strong. Raynor, ETC and Muradin being at the top of the current meta right now. At 4000, Zagara and Jaina are absolutely great. Sonya and Tass are also strong but take a little game understanding to use effectively. At the higher prices I would suggest looking at Tyrande, KT, Sylv and Johanna. Johanna is not really at the top of the meta currently, but she's a relatively forgiving hero that can be a good starting point to properly understand tanking.
I know it looks bleak and it may require hard work. I've been there. This was my first MOBA since playing a few games of Dota in WC3. My MMR initially tanked because I simply didn't understand the basics. Because of the initial MMR tank it was a long climb to reach the higher ranks and every so often I would plateau and stall out for a while. I would always say to myself that the matchmaker was giving me crap matches and that I was stuck in MMR Hell. But the truth was I was just not improving or performing well enough. You have to constantly be thinking about how you can improve and what mistakes/pitfalls you need to avoid.
You are the only constant variable in all your matches. High or low winrate, you are the only variable you can control. Good luck out there. I truly hope your experience improves.