Hmm...I've never had any kind of problem in this regard (ever since I had a sexuality I knew I was pansexual) but given your age I guess it makes sense that you might've never gotten a chance to understand yourself very well given the cultural ideas back then....I can see how an Autistic person could (like me) not notice the culture's stigma and never be "in the closet" in the first place, OR, since most Autistic people know so little about dating and have to rely so heavily on others' advice, not consider any possibilities other than those the people told them about.
But whatever. Very few people are 100% "straight" or 100% "gay"; most people will have at least one or two people they'd "bend" for, even if they never end up meeting those people. I guess being aware of your sexual urges when you look at/think about different things and being open to different possibilities is the best you can do right now.
Also, keep in mind that sexual and romantic attraction are two different things that have very interesting/odd ways of working together. You could be heteroromantic but pansexual, or homosexual but heteroromantic, or asexual but panromantic, or demisexual (only sexually attracted to people whose personalities you like) and pansexual, or aromantic but heterosexual, or any kind of permutation of all the various possibilities (there are too many to list). You may be surprised when you figure out whatever it is your genitals and feelings want.
For me, it was easy--pansexual, panromantic. I guess my higher sex drive also forced me to figure out what I wanted earlier on. But many people will end up with really "weird" sexualities and romantic attractions...have fun figuring that out! I understand that a lot of times finding people to have sex with to "test" those kinds of things is difficult for Autistic people, but masturbating can help a lot (it's really not a good idea to have sex if you haven't masturbated anyway).