Congrats on the weight loss! We could all be so lucky.
T-shirts are easy. Sophia has it right. The only thing I might add is that the difference between a 2X and a M might be huge in the shoulder width. You MIGHT want to cut the sleeve out (carefully cutting just the seam allowance) and reset it closer to the neckline. T-shirts are basically the same cut front and back--except for the neckline, which is lower in front. Try it on. Check the neckline and the way the shoulder seam lies and how far the sleeve hangs off your shoulder. If you are satisfied with the neckline and the ribbing fits well enough but the sleeve is hanging to your elbow, lay the shirt flat, measure the shoulder seam to the distance you need, decide how much you need to take off the sides for a good fit, and recut the shirt--using the contour of the side seam as a guide. Then cut the sleeve out at the seam and change the width across the shoulder and chest (about 1-1.5 inches below the front neckline). Use the old shape to guide you, but cut it closer to the center of the shirt. Find the center top of the sleeve, match it to the shoulder seam, and stitch the sleeve back in. (if the shirt is HUGE, you may have to recut the sleeve a bit. You will likely have to take some in at the underarm seam, which would require a little "tweeking" of the sleeve cap)
I know it sounds like a bunch of work, but it will be worth it if you want the shirts to fit well and they are good enough quality to make the effort worth while.
Even easier: buy a t-shirt pattern. Cut up the side seams and around the sleeve cap of your shirts. Open the underarm seam of the sleeve and lay it flat. Lay the shirt flat (front and back together--they are the same remember.) Use the pattern to recut the shirt and the sleeve. Then put the sleeve back in, sew up the side seams and you are good to go. (this will only work if you are happy with the way the neck fits. If it is too big and sloppy you will have to rebuild the whole shirt. Might as well give it away,)
If you neckline is too large and the shirt is a good weight and good quality, rebuild it but add another neckline underneath. This can be done with a contrasting ribbing wider than the one on the shirt so it comes in closer to the neck. Just sew it on the same seam that the original ribbing is sewn on. Be sure you quarter it (and the shirt neckline to match) and stretch as you sew so it will stretch when you put it on. I have several sweat shirts like this. Gives a nice layered look without the extra bulk of two shirts. Good luck!