You feel the Spirit because the Lord loves you and because your heart, despite your sins, is attuned to him enough to still feel it and be receptive to it. I, too, have felt the Spirit in my life at times when I was not worthy. The Lord loves us more than we can comprehend, of that I can testify. At times he may possibly be "slow to hear" the cries of those who have transgressed, so to speak (as mentioned in Mosiah), but he never abandons us and his hand is extended towards us always. Usually it is we who abandon him.
You do not need to choose between the Church and your boyfriend. That is a false choice and frankly, a deception the Adversary would have you be deluded in. The real choice is righteous living vs. sexual immorality.
If your boyfriend can't accept you being chaste - that is his decision.
The other thing is.....marriage was invented for a reason...................not saying you guys should just go and get hitched because you want sex. But I think a more mature perspective on your relationship is probably in order (sorry if that sounds judgmental, just trying to give point blank advice. Maybe you are already thinking of these things I'm about to mention). One that includes long term views of commitment and where things are going to go. Do you want to have sex with someone you will not marry? Will you marry him? Do you love him? Then what's stopping you? Will he join? Does he love you? What's stopping him? All pertinent questions to consider and follow through on. Heavy lifting, I know. But that's life, I suppose. I wish you the best. The Lord loves you, never forget.