1) You should resign to the fact that you will never know the truth and think about whether you can go on living with him while not having complete information. it is your call, but you should realize that you will never know the truth.

2) You clearly believe that your partner owed you a duty of sexual fidelity while being your partner. He was not your partner when you had a break. So he did not owe you any duty of sexual fidelity.

3) He does not have a duty to disclose the information about his private life during the break to you either. It is his choice. He might be having difficulties telling you to respect his privacy, and that difficulty has led him to tell you that "nothing happened", in the hope that that sort of thing would satisfy you and you would stop your inquiries. That is not lying - the problem is with your interrogating him when you should not be interrogating him.

4) She might have been lying to you when she called you to inform you that the two of them had had one or many sexual encounters. If that is what she did, she was willfully lying to you out of some kind of malice or envy - she wanted to upset or hurt you or she was unhappy that he did not stay with her but preferred you so she wanted to retaliate. This is outright intentional lying with malice as opposed to (3) above.

5) Unless she was a very close friend of yours who had always been in the habit of telling you about her intimate relations, it is weird that she would call you out of the blue to tell you about the alleged sexual encounter(s). This does not resolve the issue of whether the sexual encounter(s) happened (nothing will resolve this issue), but it speaks of some sort of malice on her end. Maybe she did sleep with him and told you that she did out of malice. Maybe she did not sleep with him but made up the story, again out of malice. At any rate, you probably should stay away from her.

6) child development has traditionally been a female-dominated field so he will continue to be surrounded by women even after he finishes school. You need to either learn to live with that or break up with him and get a boyfriend who is studying electrical engineering or is in another male-dominated field, because doing so would solve the problem of your bf' being surrounded by females in school and at work.

7) When you wrote that "He used to be insecure in our relationship because I had a couple of good guy friends, and so I tried to communicate my feelings thinking maybe he would be supportive or at least understand where I am coming from, but I feel he just dismissed my feelings", you were alluding to some sense of equity/symmetry in the relationship. You were hoping that he would draw on his experience of being insecure due to your having a couple of good guy friends and be able to view the situation from your standpoint and thus understand your feelings. It was a valid approach, but it did not work, so there is no point in pursuing it further. The next logical step is to regain the friendship of those good guy friends (or, acquire new ones, whichever is easier and more straightforward) . There are several benefits to this approach:

a) Your bf may become insecure again, and from the vantage point of his CURRENT insecurity he will be able to empathize with your CURRENT feelings better than he is able to empathize with your CURRENT feelings from the vantage point of his PAST insecurity (we already know that he cannot empathize from the vantage point of the PAST insecurity).

b) Since your guy friends were good, they will hopefully provide some entertainment and distraction so you won't torment yourself so much the way you are currently tormenting yourself.

c) Your relationship with the boyfriend will be more equitable - he will have female friends and classmates and you will have your guy friends. Equity is not necessary, but as an optional thing, rather nice.

So this seems to be the only logical next step and I recommend that you take it, review the results, and then determine next steps.