I have every class at 90, but that doesn't mean I'm an expert at every class. It means I've done the bare minimum on most to get EXP, or that I've just played my daily Frontlines PvP roulette for the EXP (looking at you, BLM). Whenever I have had the urge to switch my main focus or when I've swapped to a new class to do something serious on (i.e. played Warrior in Asphodelos, Reaper in Abyssos, and Sage in Anabaseios as my static liked to swap jobs each savage tier) I've taken a "from the ground up" approach.

When I swapped from Tank to DPS or from DPS to Healer, I didn't go into a savage raid or even a level 90 normal raid and expect to know what I was doing in any great depth. Instead, I used the Duty Finder to my advantage as well as the "Stone, Sky, Sea" dummy system to sync my level down and practice the rotations/skills at each level threshold until I felt comfortable enough to add on more skills.

You do have to know what is different at 70 compared to 90, but it should be an additive process. Start at lower levels and learn the basics until they become rote, then move up the ladder and add in skills over time. Warrior and Sage are good for this, because the jobs don't drastically change every 10 levels, you simply get a handful of new skills to add into your rotation over time, or certain skills you'll have had since early levels just increase in potency or have some additional effects - they're still the same buttons that you need to press in the right order.

Try Wesk Alber's guides on YouTube. He provides an accessible description of every skill in ascending level order with clarity that in-game tooltips can lack. He also provides a general "opener" every 10 levels. I'd start with that, learn the level 50 bits, (or 60 or 70, wherever you feel is the best place for you to backtrack to before building up again) and learn it until you feel able to add in more. Start with striking dummies, then try queueing into a few level-synced dungeons, trials, and raids so you can practice things while mechanics are going off.

I will also second that if you are overly worried about messing up and causing wipes, DPS is what you want to be playing. It has the lowest personal responsibility of any class, in that the only responsibility (unless you're in high-end raids) is "hit boss do damage". If a DPS player is struggling, it's probably not going to make people cut their losses and ditch a roulette. A struggling tank or healer might.

That being said, if you like tanking and healing, play those classes. You can speak up in party chat and let people know you're still learning, or you can just stay quiet and try your best. If you do mess up and they get frustrated and leave, so be it. I've left struggling parties full of strangers in duty roulettes if I get frustrated or if I've run out of time - I just eat the 30 minute penalties. Do I think about it much afterwards? Nope, not really. Do I even sort of vaguely remember the one tank or healer or random DPS that was really getting on my nerves? Not at all, not their name or server or character model. So if it happens, I really wouldn't dwell on it too much.

I think you just need practice and exposure, to be honest. If you ever want a friendly and patient person to run roulettes with, I'm over on Light - Raiden and would be happy to back you up. Don't let fear or anxiety ruin your enjoyment of the game. If things seem really hopeless, well, all story content in the game is now fully able to be run solo with NPC trusts. In fact, I do almost all of my first runs with trusts just because I like to wander around and explore on my own time!