Well, you are an adult (it's in the acronym) and that sometimes means being sexually aroused. You've only got a couple options when it happens. You can ignore it and wait for it to pass, or you can indulge in it. The latter is typically faster to get through with it, but if it upsets you, you're under no obligation to engage in that sort of pleasure when you're trying to regress. It's worth noting that babies can and do experience arousal and have all those body parts working, it's just not the same thing as arousal and orgasm after puberty. But there's no reason that getting an erection has to pull you out of little headspace.
I am sorry that you find it horrible though. And perhaps it may not be helpful to just tell you to change your mindset about it. However, you don't have full control over the things that do and don't arouse you. If we had that sort of control, I very much doubt those of us who wear diapers primarily for sexual reasons would have gone down a last when we were young and checked "aroused by diapers" as one of our options, if you see what I mean. So, while you do have control about whether to satisfy that arousal, your only option for the reaction is to learn that it's a natural part of who you are and to come to some acceptance with it.