Hello and welcome @WelshDragonette 
If you grip quite tightly when masturbating then that can make you become more reliant on that firmness to ejaculate (and any vagina is going to struggle to grip as tight as a hand ). You can ‘reset’ yourself by abstaining from any firm grip type stimulation for a few months.
If you can work out what it is about the position you use that works you may be able to find a few variations in the Positions of the Week topic? It’s searchable too, so you can put some keywords in the Search and tick ‘search this topic’, and it should give you a few results just from that topic. 
Kegel exercises can help with vaginal strength. I’d be surprised if this was the problem here, but doing a few of those may help with your confidence?
It’s a bit of a rambling reply, but I hope you find something that works.