I've recently learned that you cannot make friendships last forever. having recently lost three of my best and longest friendships. One became so clingy and demanding that I couldn't cope with the amount of time she was demanding, and I had to withdraw. The next got a girlfriend and I became surplus to requirements. The next had depression and decided that I was the enemy, and laid into me because she was jealous that I had worked on a project with someone else. These people changed. The friendships we had 20 years ago, or 5 years ago were the ones that suited us both at that time. Then things change, and people change, and the relationship changes too. Sometimes this is okay and other times it becomes too much to sustain. It has been really hard but I have had to accept that it is just as important to seek new friends throughout life as it is to maintain the existing ones. Be a good friend, be the friend you would want to have. And then if it goes t**s up, at least you know you did your best.