I've been in a relationship for 3 years. We met though work and although I never felt a real spark, he was a really good guy and so I gave it a chance. And it grew into a solid, dependable, warm relationship.
He was technically my boss when I started at the company but I transferred to another department after 6 months. Then I got an email out of the blue from him asking me out. There was nothing to say that we couldn't date as we were no longer working together and our relationship went from strength to strength, eventually moving in together a year later.
I do love him, and as I said earlier, he is a good man. He's a couple of years younger than me though and less experienced relationship wise. He spent the most of his 20s building up his career, maybe at the expense of his personal life.
He's not one for romance though and if I'm honest, this has been an issue right from the start. Any attempt by me to be spontaneous and romantic is met with a pause, hesitation then the moment's gone and I come back down to earth with a bump. I've tried gently trying to bring it out of him, nurture a romantic environment but I have come to believe that he just isn't able to reciprocate. Because I do feel some kind of romance in him, even though it is very inhibited. Or maybe he just doesn't feel it enough for me. 
Sex has dwindled to practically nothing, we never had much of an active sex relationship to begin with but the past 18 months or so, I'm lucky if I get any sex at all. I can count on one hand how many times we've DTD this year.
But that all said, he is safe and dependable and I care for him very much.
I have initiated conversations about where he thinks our relationship is going, and I've been quite frank about being frustrated by our lack of sex life and romance. But every time I suggest taking a break, he gets very emotional and promises to put more effort in. And I do believe him. As I said, I do feel some passion in him deep down. But nothing really changes and I just think this is who is and maybe this is just an area of our relationship that will never be amazing. There are other important areas that do work though, such as trust, respect and honesty. Values that I treasure above and beyond all else.
Recently, I went to a University reunion. And there was an old friend, Mark, who'd been in his postgrad year when I was a fresher. We were great friends for years and nearly got together - but after a week of dating as a couple (rather than hanging out as friends) I got a job abroad and we decided that a long distance relationship so early on wasn't going to work. We liked each other enough as friends to want to remain so and didn't want the potential complications of a relationship to spoil what we had.
And for years we stayed in touch periodically. Whilst abroad, I got into a serious relationship that lasted 5 years and during this period, out of respect for my then boyfriend, we limited contact although we'd occasionally send each other friendly Facebook messages on birthdays, christmas etc.
When my ex and I broke up, I moved back to the UK and got back in touch with Mark to see if he wanted to hang out again. But he'd just started seeing a woman who he ended up being engaged to. Needless to say, out of respect for his relationship we didn't have contact during this time.
Then his engagement ended and, yes, you've guessed it - by then I had met my DP!
So, back to the present now and the reunion a fortnight ago. It was so good to see Mark again and we had such a laugh reminiscing over old times. We went on to have dinner and I suddenly felt guilty, like I was cheating on my DP and so I made my excuses and went home.
Over the next few days, we've been texting and having very honest conversations. I've made it very clear that I have no intention of having an affair, although some might say that I'm already some sort of emotional one.
It's just that the time has come, I think both feel it, to be honest about our feelings in a mature and sensible way. Put any ideas about being together away for good or to finally take the leap of faith and take the chance.
He has told me that he loves me. Always has. That no-one has come close to the way he feels when we're together. I feel the same.
But what about my DP? I know I have to tell him what's been happening this past couple of weeks. We are not currently living together as he's in Ireland (on a six month contract) so we only see each other every other weekend.
We own this house together, have a joint mortgage and bank account and as far as our families are concerned, we're as good as married. It would cause no end of a scandal if I were to leave him for another man.
I'm seeing him later tonight.
How and what should I tell him and what should I do about Mark?
Feeling confused and it's not made easier by the fact that I can't stop thinking about Mark and each time I do, I feel butterflies and can't stop smiling in the soppiest way. 
Thanks so much for taking the time to read this long OP!