The anemia issue is of course the biggest limiting factor. Depending on the nature of the problem (and certainly if you're not going to compete), it might be possible to get a legal therapeutic use exemption for erythropoietin and/or intravenous iron infusion. Only a doctor could tell you if either of those is called for or would be beneficial.

That aside, from the riding and conditioning perspectives only, I think I know what the problem might be. You started riding with a group from basically the outset of your cycling experience and got pretty good pretty quick at hanging onto a reasonably brisk pace and even taking some pulls at the same pace. But this doesn't prepare you for a sustained effort at the wattage the best riders in this group are able to put out. Let's say you're somewhat comfortable at a power output of 180 watts in the draft and can even do 250 or more for a brief turn at the front, peeling off when you start to creep into the red zone. That will allow you to hang for awhile in the 25 mph range. You can also attack a short hill at the higher wattage and not be done for, recovering on a following downslope (similar to taking a pull and drifting back). This temporarily gives you the impression that you're at the same overall level as the rest of the group. And hanging in at 25 mph certainly isn't shabby at all, especially for a noob. But when you have to sustain a wattage that's in that red zone of yours and you don't have the option of drifting back into the draft to recover, that's when you crack. If a climb requires you to generate 215 watts for 5 minutes just to stay on the wheel of the next-to-last guy in the group, it's going to be far enough above your threshold for too long, despite the fact that you might be able put out over 250 watts for a very short time and then recover. Ignoring the anemia issue (which is frankly a huge issue), some of this comes down to conditioning and climbing technique and some of it is probably the psychological burden of knowing that on the hill, you can't drift back into the draft to recover when you feel the hurt starting to come on. There's no place to hide. It's just you against gravity and you have to keep that hard effort going longer than you ever need to in the paceline.

Another thing at play is that most riders in that group are probably way more experienced and fitter and are probably pacing themselves a lot better and feel quite a bit easier than you're feeling on the flats. So when it's time for every man to put out his own wattage for a good while on the climb, with little to no benefit of the draft, they're not as far in the hole as you are at the base of the hill. Despite the fact that you think you're doing fine and holding your own on the flats, they're feeling even less distress and are ready to pick up the overall effort even more on a long hill.

To get better, spend some time spinning faster in easier gears on the longest hills you can find that aren't so brutal that you can remain fairly comfortable for most of the climb, tinkering around with pedaling technique and hand positions without laboring. Also do repeats on hills that are just a tiny bit longer than you can currently power over, again practicing different techniques before really going to the well on them. And occasionally hammer some steeper hills that are short enough to be in your physical and psychological wheelhouse, but do a few reps of them so that you hurt a bit by the end. The technique, comfort and fitness for climbing should improve, and with it, the confidence will increase. Once the hills (and speeds on them) that are barely doable become quite doable and they are no longer daunting, try going faster on the longer climbs. Give yourself time to find your climbing legs - it might even take a few years before it starts to really click. You might never get great at climbing (not everyone does), but you'll get better, and riding will be more fun when you don't dread the hills and you know you can conquer them at speeds that were once impossible.