I'll do my best to try and answer your question but please keep in mind: I am a born female, identify as female however I do have a lot of m2f and f2m friends, so I will do my best (until someone with a better answer) comes along! Maybe P'Gell will stop by and give you some insight!

First of all, from knowing this alone, it could be possible that it's just part of the body dysphoria. It could be psychological. You also may need to stretch. Not trying to get too terribly personal, but I know that a friend of mine who is also in the same boat, never liked to masturbate by inserting toys. He said that he would rub the clit area, but never insert toys. If you haven't tried this, maybe you should start with a really slender toy and work your way up to larger toys. Even fingering can help.

You can also perhaps try some of the sprays or creams here on Eden which loosen up the vagina.

Also, an ex girlfriend of mine (she was m2f) who had just had the surgery (she kept bragging about her brand new tight vagina lol) had to loosen up by taking things like bullets and inserting just the tips in, because she was so tight. You can maybe try that?

I really hope that at least helps a little without knowing the full backstory to what's going on it may be tough to answer.