@TheHappySpaceman
Those zines that replied to you are ones you should keep submitting to. If they replied to you, then you have a relationship with them, and that can help over time get you small pieces.
I don’t know what the zines are, but try doing cover art for stories within them, or “Action Scenes” from stories that are on-going - those kinds of little one-off submissions can really build up a pool of art that you can use to build a portfolio, and single-use images are often a LOT easier to sell than any kind of ongoing story or involvement.
Just keep going. If you like drawing, draw, and if you like a zine, keep submitting pieces to them.
Eventually one of them might pick up one of your pieces for an action shot or a one-page, or even a cover or an ad.
And if it helps, look at the biographies of your favorite artists. Look at how they deal with rejection. Because anyone who is a professional artist today has been rejected SO VERY VERY VERY MUCH. It’s just a part of the job, and … if I may be so bold … the lifestyle.
For me, I draw because it keeps me sane. If I don’t draw, I don’t feel right. So I draw.
Selling pieces and getting published is just a happy coincidence.