Harsh criticism will not help you as much as you seem to think. You have skills, obviously. You need to practice more, and harder – I see some pieces dated from 2015 (Groot and the zombie): if you didn't do much more than what you show here since then, well, here is an obvious problem. Try to find the time to do one drawing a day, or at least one sketch.
Otherwise, I'd avice you to follow a course on anatomy, as it is your most blatant flaw right now. The muscles of your characters are almost all wrong, which lead to awkward poses and weird proportions. Such a course would also greatly help you to know where and how to place your shadows better, as you already have a good instinct for this – improving on it will not hurt as the light commands the mood of a piece more often than not.
Also, work on your rendering techniques. Learn to imitate materials or simply to include texture effects in overlay and other blending modes. Such a trick is not only a great time saver but also adds tremendous depth and complexity to a piece. This will be especially useful if you want to go for a concept artist career. Don't hesitate to work from photos and other references for fabrics and sheets. It is mandatory to know how they drape around the body and various volumes. You can also take pics of yourself or your friends with your phone...
My last advice would be to know what you want to do, and to know it ASAP.
For comic covers, you'll need to improve immensely on both your renderings and your anatomy: this will require the most work but also may be the job which will reward you the best money wise.
T-shirt art demands a strong graphic style and yours is still in development though you show some interesting things: your skulls, your Cthulhu-like monster, your heart, your dark lady with the knife,... are rather promising – you need to find and develop a specific and unique facture which will also be simple enough, or rather cost-effective to print on fabrics (did you notice that most t-shirt arts are done with flat colors?).
Concept arts is the simplest to do, contrarily to what one may think based on the pieces shown on big sites such as Artstation and others, but require a lot of imagination and creativity, for which I don't see a strong talent in the art you show here...
One last thing. Avoid showing the same piece twice in a portfolio, even with variations. The Daredevil portrait, the heart, the blue zombie,... Pick the one you like the most for each of those and show this only one. Quality vs quantity.
Keep up the good work!
