I can't top anymore

So over the past few months I have found my willingness to top my boyfriend just vanish, I never liked doing it beofre due certen physical problems I have and his size, but I could easily do it for him if he wanted to or I was in the mood.But now I just don't want to I feel like I have become and exclusive bottom, I am unsure if this is connected to me questioning my gender over the past few months but as of late whenever it comes to me topping him I can't get hard, I physically can't do it, the only way I can is to look at porn before hand and then use a ring to maintain my hard state.I am 20 so I don't be it is due to my age and whenever it comes to being alone and masturbation by my self I can get hard and perform no problem it only happens when it comes to topping.