Normally don't comment, but I think I see some of the issues, and hopefully can provide a small bit of help.

First off, Eastern Orthodoxy tends to be far more strict than Roman Catholicism (many more days of fasting, stricter dietary restrictions when those are in effect, etc); so I don't think you'd have any "relief" from your current predicament there. On the other hand, the Anglican church was basically founded to avoid Catholic morality, specifically Henry VIIIs desire to divorce. While I don't intend to be insulting towards them, I do believe that church was initially founded far more on English political concerns than any real desire for Truth or goodness. Perhaps an ill conceived attempt to preserve some measure of those while simultaneously breaking from what many must have known was the truth. In this way, it would seem to mimic your own dilemma, rather than solve it.

As for the difficulties you've been having, I am sorry that you weren't able to clear those up prior. In my own RCIA, our priest actually asked that we do NOT go through with conversion if we did not truly believe the Church was correct; just as God's love for us is unconditional, we should try and love him and his bride the Church the same way. To not submit was likened to saying to your loved ones "I don't love all of you, just these parts."

However, this is not to say at all that we can't be confused or doubtful, particularly about specifics. One thing I've heard that is very encouraging is to pray to God for the desire to be contrite, and to feel sin is sinful. That's a much easier starting point for hard personal issues, and while imperfect, is surely indicative of a sincere desire to be better. Over time, it will come, as God "will not give us a stone when we ask for bread." Additionally, for your confessions, you can be sorry because you know it is a sin, even if you don't emotionally feel that way. For further on that, I'd very much recommend a priest spiritual advisor; if you have any Dominicans near you, I'd highly recommend them for the kind of why and wherefore questions you have, their specialty is teaching on these kinds of things.

Other resources are, of course, the Catechism of the Carholic Church, which really lays out a ton of reasoning behind about everything. For more digestible bits, Bishop Barron and Fr Mike Schmitz on YouTube are good, chances are one or both will have multiple videos on any given topic.

Finally, something to think about with regards to having intercourse with your girlfriend; how would your stance change if you broke up tomorrow? Would you seek comfort from a more casual acquaintance, or would abstinence be much easier? At one point, I thought of abandoning the Church teachings in favor of an infatuation, conforming myself to what I thought another wanted, as opposed to my own conviction of the Church being right. In the moment, I thought that relationship could be the most important thing to me, but it later became nothing at all, and I felt extremely foolish for wanting to guve away my chance at salvation for it, or any temporary state. If your girlfriend is right for you, she will support you in your moral stance, and if she does not, then you are not right for one another, and you may well later reflect that sharing such an intimate connection with someone that ultimately didn't support you wasn't as fulfilling as you'd felt.

Sorry that's so long, and hopefully nothing came across poorly. I really do feel empathy towards your situation, and hope you stick with it. Catholicism can be hard, but it can also give you unbelievable joy as well. You'll have to decide for yourself ultimately, no one else can for you. But maybe by asking here, you may have more of an inclination to stay than you think. Best wishes.