He didn't trust Akshara, that's a seperate issue..but whatever he said did make complete sense..Naitik told Akshara that before her, there was no girl in his life , she was the first one and he decided that she will be the only one....When he never experienced that emotion, that love before, then it's hard for him to understand Akshara's state of mind...it's human nature that you can never understand others pain till the time you go through the same experience...a casual affair or a crush wud have made him understand the difference between "liking" and "true love"
Secondly, the intensity with which he loved Akshara, he expected the same from her..it's very natural..that when you love somebody with so much passion , with your heart and soul, then the "thought" that he/she loved someone else is scary and can create insecurity in anybody's mind..I tried hard to understand this fact & it did ring a few bells in my mind...If my husband wud have told me tht he loved someone else seriously before me, may be i wouldn't have overreacted the way Naitik did, i wouldn't have felt jealous of her or get insecure but somewhere inside i wud have felt really bad..tht's the effect of love ...after marriage if a wife tells her husband that she had a past , well past is past, you cannot change it but it WAS there..she lived it..will it not affect her husband in anyway? ideally, don't you think you would've said it in ur mind "oh i wish i was the only one"
Another point is that Rituraj angle was not important here...like we all assumed...that Naitik is overreacting bec. it's Rituraj , the guy he hates, and not any other random guy ! I was amazed when Naitik said "shayad aap usko chahne bhi lagi hongi" , this proves that it doesn't matter to him that she liked Rituraj but what was important was that there was somebody who Akshara liked or loved..who the guy was, how deep that feeling was or it was mutual or no was out of question...
When Naitik asked her the question in the ice cream parlour "how many guys did you reject before me?" , at that time i'd said that the way Naitik's nature is, he should not know anything about Rituraj...that question was very tricky..and it says a lot about Naitik's nature...he always assumed that Akshara rejected xyz but he never expected that some random guy rejected her and that she liked him...tht's why "who rejected who" became so important for him...tht's why he assumed earlier that Akshara rejected Rituraj..
Yes, he misunderstood that therez someone else in Akshara's heart, not him..but taking into consideration his sensitive & self obsessed nature, the fact that he can never think of "sharing" anything which is important to him made him think that way ! He just needed tht small reassurance which he didn't get when he needed it the most...those negative thoughts spiralled into insecurity and low self worth which made him behave the way he did..