Just weighing in because I've been there. Took me about a year to get out of that position. Now I run one of the more successful linkshells on the server.
Here's what I did:
Level relevant jobs (BLU, WHM, GEO, RDM, PLD in my case)
Start gearing those jobs based on guides found here on FFXIAH. You can solo to a decent gear level without problems.
Learn how to play those jobs
It might seem nonsensical, but you need to actually learn how to support a party if you're going to play support. Learn all the little idiosyncrasies for each job. You learn this by reading, talking with experienced players, and actually generating the experience yourself.
Last thing is be prepared to put the time in. Chances are you'll get called bad at some point and actual progression comes slowly.
Some players help new/returning players all the time - it's just about making friends and talking to people. You see someone with a badass AG setup idling in town and you play that job? Strike up a conversation.
I personally help new/returning players, all I ask is that they gear a job that will help me eventually lol. WHM is my go-to when asking people because WHM is easy to heal on and is relatively easy to gear.
P.S. Ambu is annoying this month. Only way you're getting into any efficient party is being a really geared COR, RNG, SMN, GEO, or PLD,RUN.