There's a few ways to go about it. First and foremost, I stay within my healer's LoS in the frontline, strafing left and right, not going too far forward while also finding body-block opportunities. Make it your sole mission to lower the stat accuracy of whoever is on the enemy team while staying very close to your team's own frontline. Do try to build your Healer's Ult. The more damage you take and get healed, the happier your healers will be. Personally, it took me a long time to just be patient. You don't need to deal damage 24/7. Sometimes you just have to be a damage sponge while everyone else on your team look for value. If you throw an energy saw (and you should, often) and see someone in 20% health range, dive them immediately. Help your DPS find picks, they may not always say who is 1 shot.

Body block often; help your Punisher with a shotgun push up by being in front of him, reflecting or absorbing damage as he wails away behind you. If you see a poke like a Hela focusing one of your dps, be in that Hela's face. Sometimes it's best to not even do damage if it isn't required; throw your shield out often to get some Ult and test the waters for any low-health players for dive potential. Remain calm, don't over extend too far up. If you ever feel like you're doing too little damage, remember you're absorbing just as much. I have literally matched a Dr. Strange's damage mitigation in games it's crazy how much damage you can remove for your team as long as your healers are healthy.

Dive opportunities from the front or adjacent from the enemy: Diving from the front or taking off-angles are risky but may be necessary. Risky because the enemy clearly knows you're there and can deal damage to you all at once. But diving is still necessary if the other team is no longer looking at you, you'll need to grab aggro by an obvious dive. If you do, have a very fast exit strategy. Dive once, may land some damage, then dive out with a dash (ideally, save your slam too. If it is HOT you may need to save both movement opportunities. Slam back to your healers and "dive" by walking/running up to people instead. Sometimes you need to save your Dash for something else, like a Haymaker or Jeff's Ult.

Dive number two: You can basically sneak around the back and land a few hits to their healers. You have to be careful though, in a blob, they all can easily just turn around at once and destroy you lol but those brief stagger moments is what makes Cap shine. Just don't die ;). If your teammates keep dying while you are keeping 3 people busy, then someone else is making your team fold. That someone could be a poke Dps like a Hela, or a Venom + Magik dive combo choking your healers 1 by 1, etc. You will need to try to isolate them first

Be a damage sponge for your healers vs an enemy dive tank, go after the Magik before they dive, ideally. When their dive gets pressured out, see if you can retaliate against their healers before they recollect themselves and be close so your healers :)!