It's a game of competitive misery, OP.
You're still recovering, and dealing with the limitations that weren't there before, which is hard.
Your colleagues are dealing with their normal workloads, and still picking up some of yours. They're overworked and tired.
None of you are unreasonable to feel angry, sad and/or frustrated at the cards you've been dealt. The problem is, management need to recognise the situation that has arisen and to deal with it.
After 18 months off, your colleagues probably know something's happened to you, so working out that you need reasonable adjustments shouldn't be difficult, even if you haven't shared any details. However, after 18 months of covering for you, they might be past the point of caring. Yes, they had cover for some of that time, but not all of that time, and you don't know how good or bad that person was.
I think all you can do is speak to management to explain that you're coping with the phased return, but do not anticipate returning to full-time hours for a long time, so you wondered if there were any plans to recruit given you can see how busy your colleagues are.