A pelvis should normally be sorted in 3-4 sessions, thereafter (to my opinion) you should use exercise to strengthen your core before further mobilisations should take place. The reason is simple: within 3-4 sessions it should be aligned and no misalignment should take place. In case you continue to mis align it is because of the joints are to loose and do not tighten up by themselves. The idea of Pilates is good, but you have to do clinical Pilates and not in a sports studio.Reason is that none clinical is (depending on the instructor) not precise and people tend to overshoot the goal of stabilisation and tend to use every mscle in the body to compensate (human habit to use whatever we have).
Have you done the basics of Pilates? This is to learn to turn on and off core muscles, to increase their tension depending on the load and so on. Another thing is that most likely your Gluts do not function very well (you need them to stabilise your pelvis and to allow , at least the Hams, muscles of the legs to lengthen (you might be asking muscles to multi task). Check them with a clam: lie on your side, knees flexed 90dgrs, hips 45dgrs, keep pelvis in neutral, back slightly arched back, tighten core and open legs (=external rotation of hip). compare left and right, the external rotation should be equal and smooth movement without tendencies as to compensate by rotating you body (I suppose you can find some examples on the internet)
In order to find out the likelyhood of neural tension: do a straight leg raise (SLR) as a dynamic Hamstrings stretch (Hold thigh with both hands whilst lying on back then extend knee, compare left with right) In case there is a significant difference with the least range of movement on left (cannot extend knee, with same sensation, on left as much as right) tha the reason will most likely be neural tension. Having said this I suppose you need someone to test it for you. See also Slump test, but again you might need someone to do it.
Hope this helps.