I resist the temptation to text/call as I know staying strong and moving forward with my life is the only way.
This is the only action to take, hard though it is. It
will get easier, but it will take time and a strong resolve not to give it 'one more try'. It is very easy to get back on the toxic roller coaster even though we know the ending will always be the same.
My own BPD relationship was also 4 years and I chased her every time she broke up with me. It became a game in the end, as she knew I'd still chase her so really I was empowering her actions. Eventually I realised that nothing was going to change and that this would be the rest of my life if I allowed it to continue. The mental toll it take out of you, as I'm sure you already know, was massive.
What you see over the years is always what you will get; we can't change them. We can try and work with them and adapt our lives - at our cost - to them, always walking on eggshells and waiting for the next explosion to happen but who wants that?
As HoratioX said, the person we met was never a real person. It's not as though we met someone totally genuine and lost them through our own fault, we met a chamelon who will change to suit the next person they attach themselves to, with the same inevitable conflict. We can get over this, put it down to experience, watch out for the signs in future partners and find someone more mentally healthy. Sadly, they can't.
Stay strong, use family and friends for support and occupy your time as much as possible with hobbies, etc. No harm in still thinking of your ex - I still do - but know that it was never going to be a viable relationship.
Best wishes