I lose games and I don't know why because I can't replay games.

I like to record my games (using external software) and rewatch them. It gives me a chance to analyze my gameplay outside of the heat of battle. I get the benefit of hindsight so I can see if I made the correct calls or not. Every time I see something that I did wrong, I make a mental note of it and think about how I could have done better.

For example, recently I noticed that when I'm playing as Soldier, my team often gets wiped and I'm the only one alive. A player in a bad mindset would tell themselves that, in these situations, they had terrible teammates. Instead, I decided that this happens because I'm playing too far back and not providing enough cover fire for my tank/support. So I started playing more aggressively in certain situations and saw big improvements (and climbed ranks as a result).

They hide all statistics from me, I mostly play DPS and get Gold/ Silver medals, and I feel like that the lost wasn't my fault but then I keep losing.

Even before I made that change, I would typically get 2-3 gold medals as Soldier, and it wasn't unusual to have 5 medal games. A lot of players stop here and say to themselves that they're doing the best on the team so it must be bad teammates. But that's a mindset that prevents growth.

Regardless of how you're doing relative to your teammates, if you're not winning games, then there is something you could be doing better. Of course you'll have games where it actually was your teammates' fault but that's out of your control. Focus on improving your own weaknesses and results will follow.