there are no easy answers to your question about how to deal with your feelings, but I think you know that. Going to psychologists and counsellors can be very helpful to understand where feelings come from, but I know what you mean about not being told how to actually change those feelings. I would start by looking into something called neuroplasticity if you just want a factual sense of how to change the habits of how you think. This will give you a general framework to begin with.
Breaking it down to a very basic level, you cannot think your way into a new state of mind (it's like trying to use a toolkit with broken tools in it, to fix the broken toolkit itself - you will never magically be able to do it!). What you have to do is act as if you feel differently. When you do something over and over again that goes against the grain of your established thinking process, you start to create a new neural pathway - which becomes more instinctive the more you use it. The link I've given you says it's like going off the beaten track in a wood. The first time you do it, you might feel like you're lost. But if you go the same way every day for a month, you'll have created a new pathway that is visible and easier to follow.
It's not going to wipe out the way you've thought and behaved for 20 years, but it will become easier in time. Going off the beaten track for you would mean buying presents even when you don't think you should/can or want to. Doing this every time there is a reason to. It may feel strange for a few years even, but one day you'll find it comes more naturally, and your feelings will have changed. You'll no longer be 'off the beaten track' - you'll be walking down a new, but more familiar path.
Obviously there are a lot of things going on for you that are more than just 'we don't buy presents'. But if tackling everything leaves you feeling paralysed, it could just be the starting point you need. BTW - don't just buy presents for your DP. Try buying one for yourself, and see where that takes you. Building self-esteem works the same way, you have to treat yourself nicely (many, many times and many, many ways) through your actions before you feel significantly different. But then, suddenly, you do wake up with a totally new perspective.
You've actually done this before, you just don't remember it because no-one told you that it was what you were doing. When you recovered from anorexia (to whatever degree you feel that you did) you had to begin to do something that felt impossible/frightening and alien - you had to eat more. But over time, you did just that, until you reached a point where the new 'normal' was different enough from anorexia for you to no longer be ill. That doesn't mean you can't remember being ill, or that it is erased from your life story - but it no longer dominates every day, right? That's what you can do with your relationships as well.
sorry for the thread splurge I just hope I've made sense and it helps to think about this as something you have succeeded at before. Good luck x