TL;DR- I typed all of this for you…
Marvel is Hard, I know how you feel. I was there too at one point. Marvel 3 was my first fighting game and the drive to improve as well as the hype and fun the game brings is what keeps me playing. If you don’t feel these things while playing even while losing, then I don’t think the game is for you. This game requires you have certain “Drive” or 'Will" to keep playing, I remember when I first started I played guys better than me over and over again regardless of how many times I lost, 0-10, 0-20, 0-50, doesn’t matter I’d keep playing until I won and even when I did win I’d set a new goal for myself to keep playing until I won one more! Borderline insanity if you ask me.
This may sound crazy It took me a about roughly a year to start winning consistently and become decent at Marvel, throughout that time I Picked up and dropped characters and scrapped teams almost every other week until I found the right team for me.
This is very important, picking your team is a process sure you can just say “I’m just going to pick my favorite characters from my favorite series and make it work” but at the same time just because they are your favorite characters doesn’t necessarily mean they are RIGHT FOR YOU! You will feel a certain kind of connection with the characters that are right for you almost as if you were meant to play that character, it feels natural. For a long time I played Zero because he was my favorite character, I learned all the new tech and understandings of the character but I never REALLY felt comfortable with him and because of that I lost a lot of games. Also never refuse to try a character just because they’re top tier. That’s the kind of thinking that’ll hold you back. If you try out Morrigan, Doom, Vergil and you find out that this team really jives with you then learn it!! Don’t set limitations on yourself just because! A lot of people also try to avoid teams that they’ve seen others, especially top players use. Don’t be like that, if you think Justin Wong’s team might work for you then go for it and give it a try!
Besides Team and Character selection. Training Mode and Offline against either humans or CPU’s is really key to making yourself better, when starting out most of your time should be spent here. If you don’t have a friend to play with, Playing against the CPU will help you learn the game. Learning how to move, Block, hit confirm, and open up your opponent are all things that the CPU can help you accomplish as well as improve your execution in the heat of a match. Training mode is for combos and testing out your mixups.
After your training mode time is done take yourself online or offline and play against other people. If you are playing online, keep your rank on beginner and stick to player matches. Stay out of ranked and Lobbies. Player matches provide for potentially long sets and because of this will help you learn how to fight certain characters and learn matchups through prolonged play as well as pick up on certain habits a lot of players have and counter them(Spencers Air Forward Zip, Grab punish. Vergils Stinger->Rapid Slash). Player matches are also a lax environment to learn since losing here doesn’t really mean anything, so stop worrying.
Also take a break from playing once in a while and Study up, and watch gameplay! Stream monstering is a great way to have and relax and learn how the pro’s do it. Youtube has a lot of marvel tech, tips & tricks, and tutorials from general gameplay mechanics to character specifics so search away. and of course SRK is the best place to go for any questions you have or keeping up with the new stuff that gets discovered.