Unless the first husband is a baptized Catholic and married outside the church, the first marriage is presumed valid.
HOWEVER. . .definitely approach the priest and petition for a decree of nullity. Your ‘own research’ just isn’t the same as the research of a valid tribunal with all its resources and experiences. The fact that the first wife left the marriage for a homosexual union could very well indicate that SHE had no intention of consenting to a valid marriage. . .and thus, the marriage would have been null because she did not give valid consent. In such a case, once the decree is given, the husband and the OP would be able to have their marriage convalidated.
Certainly one should never walk away from the truth because it might be ‘difficult’ to follow. Even if the husband’s marriage turned out to be valid, there is still the option for the OP and husband to either separate (if there are no minor children to care for) or to live as ‘brother and sister’. Isn’t eternal life worth that sacrifice? After all, once in heaven we are no longer ‘married or given in marriage’.
You cannot currently go to confession because you are living in a state of sin and currently are not correcting it. (and this is not to call you names, it’s just the way things are. That you didn’t realize it was sinful at first doesn’t automatically make it not sinful, but it certainly lessens your culpability for making the error in the first place. That you are trying to find ways to join the true church is also a point in your favor. God will help you.) And remember, those Catholics who are in a state of sin and will not correct it cannot go to confession either, so this isn’t something ‘unfair’ that just affects ‘you’. Knowing that might help. You aren’t alone, you aren’t being treated badly.
And you are certainly free to do all the research you can on the faith, to try to learn the practices, to read the catechism and learn the teachings and follow them so far as you are able, without actually ‘joining’ until you are free to do so.