I understand what your saying. You have feelings for this girl and you miss her when she leaves and your hurting because she is with some other guy, 20 years older to boot. When she asks to come back, you gladly let her back in because you've been with her for, like, 5 years. And you forgot what it was like to live without her in your life. Now your scared to lose her, so you give in to what she wants in an effort to make her happy, (i.e., buying her a car) and hopefully by making her happy will cause her to stay. What you don't realize and what you don't want to accept is that her feelings of romance and/or love toward you has faded, and that you are no more than a money source for her. She knows that she can come around and that you will give in, and if she leaves, that you will let her back in. Not only does she realize that she has freedom to do whatever in this situation and you will forgive her, she also knows you are a sucker for putting up with her crap. I doubt that she has any real respect for you, bro, because of it. And remember, no girl will ever feel true romance or passion for any guy they don't respect.
And, c'mon, I'm sure that your self-respect has taken a hit, too, didn't it? I know its hard to forge ahead, especially when your used to a certain person sleeping beside you for years to suddenly be gone. Its real easy to take that person back after a few too many lonely nights, and having a daily, weekly, monthly, routine that includes that person, to suddenly disappear. Minutes can seem like hours when you go through this kind of heartache. It seems like the hurt will never go away and all you want is for the relationship to be like it once was before it all went down the crapper. And you cling onto hope that it will return to what it once was, which is why you keep taking her back.
But as hard as it is, you must accept the fact that you have to purpusely LET GO of that hope, its only hurting you. As long as you even have a drop of hope within your heart, you will never be able to move on. You are getting alot of advice saying to "forget her. Move on. Kick her to the curb!" But it isn't that easy, is it? I know it isn't. You must find some type of closure to this relationship and move on, whatever it is. This moment of closure is something you have to decide it will be. But closure and acceptance that it is over is paramount to you're getting on with your life. If these don't happen, you will still have hope, and be miserable.
If you feel pain and have feelings of hopelessness, something simple to do is this. Get a calender, or draw one up, and mark off a date a year from today. This is your magic date, something to look forward to. Why? Because no matter how bad you feel at this moment, no matter how much it may hurt at times, when the day comes that you marked off, you WILL feel better on that day!!! I guaranty it. By the time that day comes, you will have a new routine, you may have even met someone else by that time. You certainly won't be spending as much time thinking about her as you may be doing now. Set your goal to make it to that day, and once it gets here, you can think, "I was in pretty bad shape, but I'm much better now. I'm now glad I made the decision to not take her back, because I would still be getting hurt, instead of being strong like I am at this moment."
I did this myself. I was in pretty bad shape. I remember after six months I went through my first day where I didn't think about my ex once. I carried my calender around with me everywhere i went. If I was at work and I started to feel down, I would take out the calender and look at it and read it. I put two lists down on it. One list was everything I didn't like about my ex when I was with her, whatever it was. The other was everything I could think of that I could be thankful for in my life. After the day came that I marked, I looked at it and laughed, because I had been over her for at least 3 months. At that point I just kept the calender around to make it to that day. I threw it away in the trash that very day.
I hope you have the strength to move on and I wish you the best, because I know how much of hell it can be going through all that shit.