It's never fun to feel like a friend has abandoned you. The first thing you should do is not blame yourself. Unless you have done something to upset your friend, there is nothing wrong with you. 
Next, speak to your friend. If you've known them for over 10 years then you should be close enough to air any concerns you might have. Ask them gently if they would like to meet up for coffee or dinner, mention that you haven't seen each other in a while and ask how they are doing. If the response is positive, then you can mention your concerns over a cup of coffee and it will all seem a bit less daunting! If the response is negative, then maybe it's time you evaluate your friendship? Do you even want to stay friends with someone who doesn't appreciate you? Although long friendships can seem sad to abandon, sometimes you can be closer to someone you met 2 weeks ago than someone you've known your whole life. Time is a construct, you never know when you might meet your soulmate! 

Mourn the loss of a friendship by all means, but try and understand your own self-worth and how important it is to find people who love and value you for exactly who you are.