My DH was very anti at first as well. Not so much because I was a Christian, but more because I am now undergoing lay ministry training (3 year part time course).
When we first met, I wasn't much of a Christian, didn't go to church because of things that had happened in my life. I started going to church again when I had my DS (DD was 2). Still though, it wasn't a major part of my life.
When I moved here, I was still drifting a bit, searching. Then I felt God's call to ministry (a year ago now!) and everything changed dramatically. Very much like the "high" you experienced at Alpha. And yes, after the high, sometimes I'd feel I had come crashing down, and feeling low. Bit like being on a rollercoaster, really!
I think you need to acknowledge the lows as a result of a period of adjustment in your life. Sometimes there may be reasons for the lows - you may be tired, it may be bothering you your DH isn't a believer for instance. Other times it may be because you miss "the buzz" of feeling the Spirit working within you. If the last reason, there are other ways God reaches out to you. It may be through scripture, it may be through just spending time quietly, listening for him, not doing a lot. TBH, those moments when you quietly feel him can be more rewarding than the "buzz". I often feel closest to God at 3am, lying quietly in bed, and there are no other distractions. God wants you to grow as a Christian, and although he is always with you, you won't always feel him there because he wants you to stand on your own two feet. However, if you get really low, believe in him and listen for him, and (I've found) he will come to you.
As for your DH. It IS a bit like having an affair! He is probably worried about how you are changing. He proably needs reassurance that you'll still love him. For a time, I felt confused about my DH, and it really bothered me he wasn't a Christian, and that I couldn't discuss it with him. But I tried to see things from his POV, and tried to become more loving towards him, making a fuss of him, and we've grown so much closer because of it! Plus he can see how much happier I've become since I've re-found God! That probably bothered him too, because it wasn't him making me feel that happy, but he seems to have grown used to it. As for discussing things Christian - I've now got friends I can talk things through with, which is very important if you can't share with your DH.
Give yourself time and give him time. Its still all very new for you AND him!
One last word. Don't beat yourself up over "not being as good a Christian as you'd like". Its an easy thing to do, particularly when you are "on that rollercoaster"! There are SO many examples of people in the bible getting it wrong - Peter was ALWAYS putting his foot in it!
HTH. xxx
God bless
Mary