Firstly I would like to say that it is great you have now found some effective treatment, and above all that you have some insight now into your hypomania and are prepared to be open and honest about it. Acceptance and education and self managemnet is half the battle.
I wish my partner had the insight that you seem to have developed with the right medication and therapy, which is obviously all helping your brain think more clearly, and rationally,but as you experienced before, inisght often goes during the hypomanic phases especially with inadequate treatment. It can go on causing damage in relationships for years if undertreated, or untreated,and lots of personal losses, ,spiritual,social, financial and emotional ensue untill properly treated and managed. It sounds as though you are on the right road.
You asked why is bipolar hypomnia 2 so destrucive to relationships. I believe and have researched alot, and experienced it first hand, that it is becuase it generally goes underdiagnosed and undertreated if treated at all for much longer, and the depression which is often wrongly treated with antidepressants can lead on to hypomania or mania ,triggered by the antidepressants agin and again until properly diagnoses as bipolar rather than unipolar depression. It goes on for e decades, 19 years on average before individuls receive the correct treatment in some cases, and because the hypo manic side is less severe ,therefore less noticeable than acute mania, or severe or prolonged depressions, it is less likely to come to the attention of the psychiatrists early enough, and there is a much stronger tendancy for individulas to deny it. They may only present to the doctor during the depressive phases, and tend to play down any hypomanic symptoms or not even recognise them or describe them to the doctors as you said because of the genuine lack of mental insight into the hypomanic or manic phases that is common to 50% apparrently , who do not have insight, and invariable belive themselves to be vetry well when in the high mood swings, not recognising theoir behavoir at that time, and adamently resisting anyone's attempt to point it out. The longer this goes on, the more blame of others is typical,and excuses, or attempts to rationalise behaviours, or explain them away becomes a pattern, and it is harder for the individual to repair the cumulative damage of the years and the losses, which in turn the stress of which triggers further episodes. This can also impact on confidence in sustaining real relationships , perhaps for fear of the inevitable harm likely to the partner.
It need not be this way with optimum treatment, counselling and working on management of the illness together with a supportive partner.
During hypomanic and manic episodes and mixed episodes, individuals often become very dysphoric, not euphoric except maybe intially . The dysphoric type of hypomania can cause people to become exceedingly irritable, vey aggressive, insulting, absusive verbally and physically, inconsiderate, illmannered, vulgar, reckless, decietful, arrogant, cold, uncaring, totally lacking in empathy and risky in other areas, sexually and financially, and have sudden out f control rages, and take offs without saying where they are going. It is very hard for people to see this behaviour as unreasonable in themselves when it is happening because the hypomania often leads to a loss of rational thinking, and loss if judgement. You do not have to be manic to show these extremes. The only thing is that there is usually more delusional and even more extreme behaviour with mania, and the risks becomes riskier, and the behaviours more frequent and less manageable over a much quicker period of time, whereas hypomania can build quite slowly sometimes over a few weeks before really taking off, whereas mania often comes on and is full blow in 24 -48 hours, is usually noticeable and requires hospitalisation, that is , if the person does not end up in the police station first or back down in a deep depression.
Naturally with all this destructive behaviour taking place coupled with the individuals inability to see it as it is or even remember it, this will cause huge conflict in relationships, particularly if the other partner does not know you have bipolar, much about it or how to manage it or how to help you . Even with good knowledge of the condition,when they do attepmt to help and express their concern that you might be showing signs of hypomnia and suggest you seek help, they are met with an abusive, hostile response at the mere suggestion that there is anything wrong with you, followed by a refusal to communicate with them, to completely cut them off perhaps from contact, or to miantain long periods of hostility towards a caring partner even for weeks whilst the episode runs on, because of the lack of acceptance, and lack of insight of the bipolar partner. It creates tremendous conflict and tests even the best and most loving of relationships, because basically you become a completely different personality, act out of character in an extremely difficult way, and do destructive things. Acute mania is more likely to come to the attention of the psychiatrist much quicker and receive faster treatment, as it is harder to disguise, deny or dispute. Even consultant psychiatrists have been known to miss hypomania frequently which does not help those who rely on their expertise and medication adjustments early enough.
I am so happy you have found some effective treatment, and I hope your partner will respond in time, and learn as much as she can too, which would really help both of you. Joining a boploar support group can be very helpful in terms of education, support for both of you, and reduce the isolation for both. This can be a vety isolating condition.
I am still struggling on with the psychiatrists to find effective treatment for my partner who has bipolar 2 mixed and rapid cycling disorder which seems particularly hard to treat. He too has no insight during the hypomanic phases, not helped by his pdoc and team missing the blatently obvious symptoms too, as they see him only for a brief consultation time, rather than at home or regularly. He also went undiagnosed for decades for the resaons described above, and just got by, but has not been succeful in work, which has not helped, despite being highly intelligent and having 2 degrees, and has had numerous painful consequences of this very painful illness. I am not giving up yet though.
It really sounds as if there is great hope for you to get to grips with this and have much better quality real relationships now that you are learning so much and receiving more effective management. The very best of luck for the future. Julie fast.com has an excellent website which may help you and ypur partner. Listening to her previous radio shows which only started amonth or so ago may be of interest. have not missed one yet ! Ever Hopeful.