No. I don’t think this can be “mended”. I don’t even think there is something worth mending. This guy is not boyfriend material.
Please take an honest look at why it is that you are willing to stay in a relationship with a liar who lets you down often and doesn’t join with you in making good times together. You deserve far better. But as long as you stay attached to this guy, you aren’t available to find someone who can be a real friend and partner. It’s simply not true that “half a loaf is better than none.” Accepting so little just keeps you out of circulation.
I encourage you to take a big step back. Join activities where you can meet other people your age who share your interests. Doing things together will help you make friends — and perhaps find a special someone who will cherish you and treat you well.
I wish you well.