hey, season 2/3/4 Ana main here, I also dropped a ton of SR this season. Fell from ~4k peak to 3.3k lowest, but have since climbed all the way back up (after a month or so). What I learned in the process:

Season 4 is a bad time for Ana mains. Dive comps + weakened Ana DPS + Mercy mains everywhere means you can't just jam Ana into every game and expect it to work out. Mercy's heightened viability means you are now competing with Mercy mains and Mercy one-tricks for that main healer role, and often times, Mercy will be the better pick for your team. I am also finding this means a lot more matches where you have 3-4 support players fighting over who doesn't have to fill Tank/DPS-- these games are almost always losses.

What I changed: I stopped instalocking Ana and now "fill", more or less. I give the other people on my team a few seconds to pick-- if there is a Mercy player, they will almost always lock her right away, which is my cue to fill Zarya/Rein or play Lucio. If nobody locks a healer, I lock Ana, but sometimes the your team comp vs. their team comp ends up such that Mercy is a better pick, so I've been playing Mercy this season when I feel it is appropriate.

here's my MOW if you wanted to compare https://masteroverwatch.com/profile/pc/us/boots-11752