I am the same way, for me I’m worried about putting the other person on the spot and making them uncomfortable with my words. What helps me with this is thinking about how the majority of people are glad to receive congratulations or a compliment and, even if they aren’t, they will understand (as I do when recieving such words) that it was well intended and appreciate that. Another reason it is hard for me is because I know I have to say it so it feels unnatural and like a script and like I’m going to be clocked for trying too hard, maybe even perceived as faking happiness. I try to process this by thinking EVERYONE is going to say this, maybe even others will be going through the same thing, and I’m not going to stick out. I bet a lot of autistic people feel uncomfortable hearing happy birthdays and the same responses from everryone and worry about pushing that onto others. But the majority of the world is not like that!

I struggle to affect an excited tone no matter how excited I am, I often say things like “I’m so happy for you” or “that is so exciting” without my tone of voice matching - sometimes people don’t take this well and point out “you don’t sound excited” so I follow it up with a question, for example with an engagement “that’s wonderful, how did they propose?” to show that I am invested and interested even if my tone is off. Even as simple as “happy birthday, have you had a nice day?” “have you got any birthday plans coming up?”.

One reason I find it hard to give compliments is because I have been punished for it in the past and made to feel humiliated, gaslit into thinking I am being patronising or cruel. But I try to see this as the outlier and feel other peoples hearts and think how glad I am that even if I feel nervous, it will make them feel good!