I don't get it, I suck at like every multiplayer game ever made. Why am I so good at single player but so bad at multiplayer? ╥﹏╥
Single player and multiplayer are different realms. Single player, most things are predictable and can be accounted for easily (ex I feel like people thought Battletoads Turbo Tunnels were difficult when it's just route memorization, touhou shooters seem to be pattern memorization and some twitch reflexes, I can one credit clear Time Crisis because I generally know when the red guys appear, etc). Even the games with some randomness (Diablo level layouts, Rogue Legacy), there's still a general pattern you can use to get through.
You go to multiplayer, you now have something unpredictable to deal with... another player. Will they be predictable (Filia instant air dash into jHP at neutral every time) or stupid (zoning ringlet spike only from midscreen) or smart (stuff I don't react to until it's too late)? Even with co-operative games like Metal Slug, that second player suddenly messes with the usual firing patterns of the soldiers and gets you killed while playing single player is more predictable.
All that said, don't worry about the losses for now. If you actually put effort into improving (practicing motions, reviewing videos to see what you did wrong and how to improve, experimenting in practice mode, and playing lots of matches), you'll see improvement. Hell, if you get really good at reading what people are gonna do (can predict resets, knowing how to make your opponent do things they shouldn't), the game kinda starts resembling single player. Never fully, but a little.
Here's a decent video series that could help you out with understanding fighting games some more (if you think you need it)