Mojo - You sound tired and world wiry - something so many of us feel at this time of life.
I believe, in fact I know from my own personal experience, that one needs to change ones mindset at this time of life - it takes time, discipline and a will to conquer these negative feelings. HRT can only do so much. The Citalopram may be partly to blame for the feeling of being ‘flat' but you owe it to yourself and your son to really look at your life and do some changes that will improve your mood and view on life.
I'm sorry if this sounds harsh but the answer lies with you - you have choices and if this inheritance gives you financial security then this is fab - a golden opportunity to find a good away forward.
I have had some very nasty bouts of depression and have felt really low and desperate at times - I had a year of weekly CBT counselling that worked wonders for me. If you have the money to go privately then this can be great (you're not having to fit in round an NHS timetable) - do some research to find the right therapist and allow them to help you move forward in a positive way. Your GP may be able to suggest someone good.
Don't put off having the Mirena - there is a very good chance it will sort out the bleeding problems and it suits the vast majority of women who have one. If the holiday is in the next couple of weeks, why not book an appointment now to get it fitted when you get back. This would be a very positive step. Don't expect the worst.
It could be worth trying to slowly come off the Citalopram - you may not need this now? Again, a therapist would help you with this withdrawal.
Make some plans - write that list of things you haven't tried but always wanted to do. It is hard to do this without a life partner but I'm sure your husband would want you to have friends and enjoy life to the fullest - as Menomale has suggested, do a bit of dating???.
This thread you have started, expressing your current feelings, is a very positive step - you are open to advice and that is the most important thing. DG xxxxxx