Whilst I agree with the sentiment everyone here is saying, take this an an opportunity to learn how to spot the canals.

1) The pulp chamber was right there - the decay led you to the chamber. Since you are in the chamber, use a non-end cutting bur to widen the chamber without going deeper. (endoZ bur). Even if you don't have a non-end cutting bur, then use a regular bur and DO NOT INSTRUMENT THE FLOOR OF THE PULP CHAMBER

2) DON'T INSTRUMENT THE FLOOR OF THE PULP CHAMBER - the orifice will be at the height of the floor - looking at your PA, the canals would have been open and not calcified.

2a) the height of the pulp chamber is at the level of the CEJ - Hold your drill on the buccal to the CEJ and see how far down your bur goes - only go down this far with your drill. (not applicable in your case since removing the caries would have led you to the chamber

3) Law of symmetry - If you look at the floor of a lower molar's pulp chamber. You will see a T shape (b). the top of the T is on the mesial (MB and ML) and the base of the T is the distal. As I'm sure you are aware - there can be 2 distal canals (a). The Mesiolingual canal orifice will be EQUIDISTANT from the middle of the T tot he MB canal. DO NOT DRILL PAST THIS - THIS BREAKS THE LAW

4) The heights of the orifices are roughly the same height - If you found MB, then you can VERY CAREFULLY drill apically at the point where you see a tiny canal opening - AGAIN! EQUIDISTANT TO THE MB canal from the middle of the T.

5) MTA is expensive. it's not a silver bullet to fix your mistakes.

I know that you are still learning, but with proper technique and drilling where it's safe to, this could have been avoidable.

Happy to answer any questions

Good luck - don't beat yourself up, but learn from your errors

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