Good Lord, what a horrible relationship to have been stuck in for so long. It must be exhausting, not to mind soul-destroying for you. I think your opening sentence both asks AND answers your quandary. You're in love - but he's a liar. So you're in love with a guy who has no truck with the whole 'being honest and respectful to the woman he's in a relationship with' thing - which, for most self-respecting females, is Number One on the Deal Breaker List. That can't make you feel very good about yourself, now can it?
From what you've said, it seems like this has been the status quo for years, and yet you wonder why he's still lying to you about pretty much everything ever since you let him get away with his attempts at infidelity. You've been telling him in no uncertain terms for YEARS (without having to physically say anything) that you're totally fine with him being deceitful, disloyal and sneaky. Of COURSE he's going to keep doing it. Why not? There are no consequences for him whatsoever; while you get the lovely reward of tormented nights, self-doubt, distrust, and generally driving yourself up the wall as payment for your acquiescence. Who do you think got the better deal? And for how long more are you going to sentence yourself to this mindset? Is your time on this earth less important than his? Are you worth less than basic honesty and respect in a relationship? Only you know the answer to these questions. I hope you're good and angry after reading this far into the answer...
To put it simply, this problem is about you. He's not going to change, and you can't make him. The only part of this you can control is you and the standards you set for yourself in relationships. If you don't think, at the very fucking least, that you're worth basic truth and respect from someone, then you stay stuck in this mental hell-hole and brace yourself for a lifetime of misery and insecurity.
I can't tell you to stop loving him obviously, but I can tell you to try and start loving yourself a bit more. Would you let a dear friend or relative stay with a man who, by your own admission, is a liar? I'd imagine the answer is no. So why are you worth any less?
Either way, while you're still involved with this guy, I'd start by immediately calling him out on every lie he tells you to let him know you've had enough. Write each deception down, keep a detailed list, and if you're still with him in 6 months, hand that list to your best friend and ask her what she thinks. Her language may be far more colourful than mine.
Bottom line: Be nicer to yourself, prioritise your own mental health and happiness, and you will begin to command more respect and honesty from those around you.
What You Can Do Today: Tell him to cop the fuck on with the lies, or you'll start a blog called My Lying Asshole Partner - and give his name.
If you end up doing that, send me the link.
Best of Luck!