Saṃvega or something else? Integrating sammuti sacca with paramattha sacca

Although you put a lot of detail into it I don't really understand the question.

Normally if all I can say is "I don't understand the question" then that's not really an answer and ought to be posted as a comment instead, if it's even posted at all.

In this case I'm hoping that the problem might be that you don't fully understand the question either; so if I explain where it's ambiguous that might give you some insight.

1. You use the word "stable relationship" a lot.

   • In what sense are they "stable" if you keep dropping out?
   • What's a "relationship"? Do you talk? Groom each other? Play together? Share friends? Share money? Share ethics? Work together on the same projects?

2. You said, "I took it to the point where firm commitment was needed"

   • Who "needed" the firm commitment: you? Your 'partner'? Your 'society' or families? The relationship itself, somehow?
   • Is "firm commitment" meant to be understood as a euphemism for "marriage"?
   • "I took it" sounds very active/dominating/manly. Do the women take it somewhere too or is the relating all quite one-sided?

3. You said, "silly selfish samsaric satisfaction". Is that an accurate summary of how you view your relationship? Your intention for having a relationship?

   You might possibly enjoy the book which I described in this answer. It's meant to describe what the Buddha said a good/ideal relationship might be. You might prefer that as a way between extremes, i.e. not just "silly selfish samsaric satisfaction".

   Perhaps a "relationship" shouldn't be entirely "selfish", and should at least include your partner, and benefit other people (perhaps including for example your friends, parents, children, and any teachers, subordinates, bosses, etc.) too.

4. If it's true that you can "also see exactly how the relationship would fail a few years down the line" then you're more prescient than I am: I doubt I can predict how people will be (the same and or different) in a few years

5. You say "all encompassing love and compassion for everyone". What's "love" here: does it mean "desire" to have someone? Desire to please or flatter someone? Do you literally feel that for everyone, or only for potential "partners"?

6. You say "do I really love my current partner any more than the rest of the world". I don't know but is "love" all-important? You might know your partner, cooperate, and have negotiated agreements with your partner that you haven't with anyone else.

7. You say, "Confiding these feelings with the partner has never helped of course." That's a big topic all of it's own: how to confide feelings with a partner. Maybe a relationship counsellor? Maybe confide potential feelings early in a relationship, perhaps while you're 'dating' e.g. selecting a future partner?

   And, "Worse, they would very soon sense that I am not really bound down by anything, and resulting insecurity would lead to conflict." Are your partners expected to find security in your being really bound down by something?

I'm going to skip (not address) the second half of your question.

"Lapse into universal love" is an odd turn of phrase, as if "universal love" is a flaw. Maybe there's some misunderstanding there, and something doesn't really mean what you think it means?

You mentioned the "Two truths doctrine" in the title but don't say which school of Buddhism you might be from or practising.

If I can say so without being insulting, and not knowing where you're from or what your 'morals' are, your question slightly reminded me of Macho Buddhism: Gender and Sexualities in the Diamond Way (a school/form of Buddhism which I otherwise know nearly nothing about).

One more thing:

One moment wanting a relationship and the next day or week wanting to be a monk.

There are many monks, many types of monk's life in different places; but the choice between 'relationship' and 'being a monk' might be a 'false dichotomy': i.e. perhaps you could be a monk and have loving relationships, it's not an "either-or" choice. Now I don't mean sexual relationships, but if you read something like Becoming Buddha’s Disciple or look at some of the photos on that web site or read other blog entries it appears that monks have relationships and opportunities to "love" -- there's a monastic community (a Sangha) and relationships with the lay communities.

Also your section title is about "integrating" the two truths: does that imply that you're interested in the slightly later lineages of Buddhism?

Finally "figuring out exactly what to do" might be difficult in a vacuum: because what you do and what you can do are conditioned by environment, and by inter-personal relationships. Might a slightly different perspective help: thinking about who you want to be, or how you want to practice?

If you want to think about relationships (or a relationship), can you work out what you expect? Whether that's a realistic expectation? As I wrote earlier I didn't understand what you wrote about love and relationships, so I don't know. But you said, "were in quite a lot of misery when I left" so that's cause for concern?

If "relationships" are a problem might that have anything to do with 'right speech': saying too much, too little, at the wrong time?

Or, again trying to avoid a false dichotomy between "silly selfish samsara" and "wanting to be a monk" have you considered trying to be a "good" (or a better) lay-buddhist aka 'householder'? For example there's an article here on Theravada Housholder ethics which says, "the Buddha defines an upāsaka in terms of faith (śraddhā), morality (śīla), liberality (tyāga), and wisdom (prajñā)". And later on the same Wikipedia page there's a tiny paragraph about "Vajrayana perspectives" too.

Philosophical thoughts can come to the mind at anytime. You shouldn't get too excited about them as that can lead to complacency, ego and false self assessments. Simply note them rising and passing away. If you are not enlightened, you are not safe no matter how deep you feel or think.

Don't focus much on love. Usually Patikulamanasikara meditation is recommended to people with lust issues. The details are in the Visuddhimagga. You can also do Silanussati and try to perfect your virtue. One suggestion is to stay in a forest monastery for a while, stay away from women and work on taming the mind.