My dad left when I was 5 and I have always had a void in my life. I've done a lot of thinking about how that relates to polyamory, but that's probably blog material
I've had lots of crushes and loves, many of them more like those of a 12-year old, where I was more interested in the affection and the interaction with men than a sexual relationship. I tend to be attracted to men who are safe and kind and, well, "fatherly" (I don't necessarily mean older, but they just seem mature and grounded). Even my husband has learned that sometimes I just need a lap to sit in, a "soft place to land," and my lack of sexual interest at times is just a need for a different kind of nurturing for awhile. It always passes, and he tries to remember that!
Professors are usually advised not to enter into romantic relationships with their young students. If he rejects you, try not to take it personally -- he may actually be showing you the love and respect you deserve. It sounds to me like you are very vulnerable right now, and I hope he does not take advantage of that.
The other thing I am sensing is that you have a very, very big heart and a huge capacity to love. Whatever path you choose, that is cool.