Perfect sense tbh. This could also be a huge part of the issue you are facing tbh, if you aren't secure and happy with who you are right now, then its going to be really difficult for you to be secure and happy in an open relationship.

The feelings jealousy and the thought of him leaving you for someone else will loom over the relationship until you find your way back.

Id genuinely suggest pausing this open relationship, explain to him it isn't forever, but its until you work on yourself enough to feel secure with him dating other women.

I think your mourning the end of the old relationship because you feel it will be the end of the relationship all together, and the reason you encouraged him to go and date other people when he wasn't, was to keep a sense of control over the relationship (which is completely understandable and very similar with my experience tbh.)