I can sympathise with that. Im a serial apologiser and the reflex can be really strong. Having a safe word may help (have you considered the traffic light system by the way, also works well) Reassuring her your fine and telling her how much you enjoy it, both after and during, may help.
Even throwing a “please don’t stop, that feels so good” in there is a nice reassurance.
Anal wise i would recommend starting with butt plugs,( or theres an ocean of prostate toys out there too, sorry, not my forte but the forum is full of advice on those) before you work up to dildos.
For a first butt plus, i would recommend going with something silicone and 3.5” in circumference. (Read the sizes, both circumference and insertable length really matter). Also get some good quality anal lube (sliquid is my go to). Lube is really important so dont be afraid to use it.
Ton of advice on the forum about plugs too.
They do tend to feel odd for the first few times you use them, you kind of have to acclimatise to the new sensation.