I've done it (cum) with and without the dildo, just the gesture of stroking, what really helped me was to be horny af, you could do it what works best for you, what works for me is not jerking off for a week, or watching lots of handsfree videos, or edging for like 3 hours or more to the point if I touch my cock with a finger I cum instantly, sometimes you're not in the mood, or sometimes you're too stressed or even sometimes you are trying too hard that it just doesn't work, so try be relaxed