I have been in exactly the same place as you, and my relationship started pretty much the same.
If he has been lying most of his life, then he needs professional help to stop this behaviour. You will never have peace of mind in this relationship without trust. Lack of trust destroys relationships. You will spend your time questioning everything. I understand how you feel so connected to this man, and the fact that he has been able to open up to you about his problems also makes the connection feel stronger. But you cannot help him, he must decide to help himself. You may be able to sweep the lies under the rug for a while, but soon the rug will have so much stuff under it, it will all come tumbling out. At this time, i would suggest looking after yourself and keep close the support you have. If he really wants to change his life for the better, he will be prepared to seek proper help. He will probably put every excuse in the way not to do it, finances, lack of time, shame. The stories on this forum all have a familiar ring, alot of women (and men) like us, trying to have faith that things will change, and giving the CL's chance after chance, with their peace of mind totally eroded, but reading the stories, its a very hard behaviour to shake, and the few sucess stories have been put up on here, those that have been, the lucky few who accept they have problems and seek proper help. Youre story is so close to mine, the same events and feelings. I am still holding on, and in some ways am now very switched off to the situation, and hopefully my b/f is now weeks away from getting the help he needs, I just hope that in the longer term, it works, as there is nothing else left to try. I have had 8 months of one excuse after another, and, he even lied and said he had been going to therapy - once again i believed him and we got back together to find its more lies. This time, the only way i will believe it, is if i attend as well! I have been where you are many times, to be back there again many more. I wish you luck for the future.