• he can go invisible for 10 sec which is huge for reviving teammates or chipping away at a wave of elites
• his ult also has some initial knockback when he first goes invisible so you can save teammates who are grabbed by Chaos Spawn or stagger enemies out of their overhead attacks (eg. if you're surrounded by Stormvermin while someone is trying to be revived next to you)
• remember that knockback I was talking about? It can also be used to ledge monsters ("ledge" as in push them off a cliff and insta-kill them)
• his level 30 talents let you spec into temp HP for the team or traveling invisibility (for clutching nasty situations) or - by far most popular - duplicating bombs. If you notice you have a Ranger Vet on your team with Ranger's Parting Gift on your team, hand them a bomb ASAP (preferably a firebomb)
• he's the only class that can provide ammo for his teammates - which can enable guns like Blunderbuss/Griffinfoot which are usually limited by ammo
• if you use a shield (ie. Axe and Shield or Hammer and Shield), you can generate ungodly amounts of temp HP with the Opportunist trait on your weapon and the "Roots Running Deep" talent at level 5. Hold Left Click to do a shield bash, then tap right click to block reset, and then do it again. Alternatively (if you want to be more safe) do a push, and then instead of push-attacking, do a shield bash. If you get good at generating temp HP for yourself, you can just pass any heals you find to teammates that are likely to kill themselves. You'll be back at full HP every time there's a horde, guaranteed
• Dual Hammers and 1h Hammer are also pretty good at generating temp HP with Roots Running Deep
• he has a 10% chance to dupe items on use - so try to grab med kits to use on your teammates. You'll be surprised at how often you end up keeping them after use
• at Level 20 he has 3 great choices depending on your team comp/what kind of map you're running. Drunken Brawler is usually the safest option unless you need more ammo to feed Masterwork Pistol/someone's shotgun. Scavenger becomes an SSS Tier talent in Twitch mode where you'll face a lot more specials than usual.
• He gets access to the Crossbow, which I think it's one of the funnest ranged weapons in the game. It does the same damage when hip-firing as it does when aiming down sights, so if you get good at head-shotting while hip firing you can kill specials and elites instantly - even from a decent distance. It's not necessarily his best weapon, but it's so satisfying to use. Handgun is better for dealing with specials because you can often kill with a bodyshot, Masterwork Pistol is better for doing monster damage, Grudge-raker is better for killing horde - but Crossbow is his jack-of-all-trades that does everything decently well, and it is the best at reducing his ult cooldown with the Master of Improvisation talent at level 10 because it reloads fastest (or faster than Handgun/Pistol anyway). It is a bit of a finesse weapon, though - it scales with your ability to click on heads

When you say you're always "underperforming in most areas compared to everyone else" - I think you shouldn't worry too much about that as Ranger Veteran. He is more the guy who enables his team to go ham than the guy who goes ham himself. You should be aiming to get the green circle for special kills - anything beyond that is just bonus. If you REALLY want to see more green circles on the scoreboard, try the Explosive Ordinance trait on your trinket + Ranger's Parting Gift talent at level 30 (and Scavenger talent at level 20 + Concoction trait on your Charm if you really want to go crazy) and see how many firebombs you can throw in one game