Easy to say "don't feel guilty" but kind of hard to do sometimes.
If its any comfort, the average delay from onset to diagnosis is something like 8 years, so actually you can pat yourself on the back for having her diagnosed within 4! Its not just parents who "miss" the signs either - doctors frequently fail to pick up AS until its very obvious, and if doctors with medical training can't spot it, how would they expect parents to? In fact, you did do well - you took her to the doctor, you kept taking her back, you pushed for doctor visits and rheumatologist.......and you got there. Please congratulate yourself rather than beat yourself up.
Also, don't think too much about permanent damage until she has got her treatment working - you may find that what you think was "permanent" disappears completely with appropriate treatment. Some folk have had absolutely incredible results from treatment.