Hi,
While dogs love all members of their pack, they also tend to have favourites. This could be either the primary caregiver, or the person who tends to spend more quality time with him thus bonding with him in a special way. Some dogs choose the younger teens who they see as their playmates, others are mommy’s boys if they know they can manipulate her emotionally for extra food and attention.
Few others seek out the most confident parent who they can look upon as the pack leader. Often, separation from this favourite person can make the dog even more miserable. Your girlfriend is probably his favourite in the pack, and hence, he seeks comfort by sleeping on her belongings particularly, because they would contain her scent.
You can definitely make a big difference in his perspective and help him get over his sadness and misery.
For him to feel less sad about your girlfriend’s absence, he should begin to feel closer to you and be comfortable in your presence. This would mean that you need to bond with him and give him a sense of security that he misses.
Take over as his primary care taker. Take him for his regular walks and play with him whether or not your girlfriend is around. Compete with your girlfriend in showering him with the attention he needs and spend quality time with him. Doing so, you may not be able to take your girlfriends place as his favourite but you can definitely make him more comfortable with you. And if you do succeed becoming the new favourite, Kudos!!