No doubt those four layers of carcase on the sides are making it difficult to get both beads into the rim well on the opposite side.
At risk of teaching granny to suck eggs, work the beads into the well of the rim on the opposite side and tie, tape or strap the tyre and rim tightly close together at that point. Have the valve at the last point you are trying to get the tyre over the rim edge.
That should do the trick. Once on, shove the valve up to sit it's seat above the beads and centre the tyre on the rim.
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